Nonaqueous electrolyte secondary battery

ABSTRACT

A nonaqueous electrolyte secondary battery includes: a separator including a polyolefin porous film; a porous layer containing a polyvinylidene fluoride-based resin; a positive electrode plate having a capacitance falling within a specific range; and a negative electrode plate having a capacitance falling within a specific range. The polyolefin porous film has a given piercing strength, having a value of not less than 0.00 and not more than 0.54, the value being represented by the following expression: |1−T/M|, where T and M are distances at which a critical load is reached in a scratch test in which the polyolefin porous film is moved in transverse and machine directions, respectively, under a constant load of 0.1N. The polyvinylidene fluoride-based resin contains an α-form polyvinylidene fluoride-based resin in an amount of not less than 35.0 mol %.

This Nonprovisional application claims priority under 35 U.S.C. § 119 onPatent Application No. 2017-243280 filed in Japan on Dec. 19, 2017, theentire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a nonaqueous electrolyte secondarybattery.

BACKGROUND ART

Nonaqueous electrolyte secondary batteries, particularly lithium-ionsecondary batteries, have a high energy density, and are therefore inwide use as batteries for a personal computer, a mobile telephone, aportable information terminal, and the like. Such nonaqueous electrolytesecondary batteries have recently been developed as on-vehiclebatteries.

For example, Patent Literature 1 discloses a nonaqueous electrolytesecondary battery including a polyolefin porous film whose ratio of (a)a TD-critical load distance (a distance in a transverse direction atwhich distance a critical load is reached) measured in a scratch test to(b) an MD-critical load distance (a distance in a machine direction atwhich distance a critical load is reached) measured in a scratch testfalls within a certain range.

CITATION LIST Patent Literature

[Patent Literature 1]

Japanese Patent Application Publication, Tokukai, No. 2017-107848(Publication Date: Jun. 15, 2017)

SUMMARY OF INVENTION Technical Problem

Note, however, that there is room for improvement in the aboveconventional nonaqueous electrolyte secondary battery from the viewpointof a charge capacity of the nonaqueous electrolyte secondary batterywhich has been subjected to high-rate discharge. Specifically, thenonaqueous electrolyte secondary battery is required to have a strongercharge capacity characteristic after being subjected to high-ratedischarge.

An aspect of the present invention has an object to achieve a nonaqueouselectrolyte secondary battery that has an excellent charge capacitycharacteristic after being subjected to high-rate discharge.

Solution to Problem

A nonaqueous electrolyte secondary battery in accordance with a firstaspect of the present invention includes: a separator for a nonaqueouselectrolyte secondary battery (hereinafter referred to as a “nonaqueouselectrolyte secondary battery separator”) including a polyolefin porousfilm; a porous layer containing a polyvinylidene fluoride-based resin; apositive electrode plate having a capacitance of not less than 1 nF andnot more than 1000 nF per measurement area of 900 mm²; and a negativeelectrode plate having a capacitance of not less than 4 nF and not morethan 8500 nF per measurement area of 900 mm², the polyolefin porous filmhaving a piercing strength of not less than 26.0 gf/g/m², the piercingstrength being measured with respect to a weight per unit area of thepolyolefin porous film, the polyolefin porous film having a value in arange of 0.00 to 0.54, the value being represented by the followingExpression (1):

|1−T/M|  (1)

where T represents a distance at which a critical load is reached in ascratch test in which the polyolefin porous film is moved in atransverse direction under a constant load of 0.1 N, and M represents adistance at which a critical load is reached in a scratch test in whichthe polyolefin porous film is moved in a machine direction under aconstant load of 0.1 N,

-   -   the porous layer being provided between the nonaqueous        electrolyte secondary battery separator and at least one of the        positive electrode plate and the negative electrode plate, and        the polyvinylidene fluoride-based resin containing an α-form        polyvinylidene fluoride-based resin in an amount of not less        than 35.0 mol % with respect to 100 mol % of a total amount of        the α-form polyvinylidene fluoride-based resin and a β-form        polyvinylidene fluoride-based resin contained in the        polyvinylidene fluoride-based resin,    -   where the amount of the α-form polyvinylidene fluoride-based        resin contained is calculated from waveform separation of (α/2)        observed at around −78 ppm in a ¹⁹F-NMR spectrum obtained from        the porous layer, and waveform separation of {(α/2)+β} observed        at around −95 ppm in the ¹⁹F-MMR spectrum obtained from the        porous layer.

In a second aspect of the present invention, a nonaqueous electrolytesecondary battery is arranged such that, in the first aspect of thepresent invention, the positive electrode plate contains a transitionmetal oxide.

In a third aspect of the present invention, a nonaqueous electrolytesecondary battery is arranged such that, in the first or second aspectof the present invention, the negative electrode plate containsgraphite.

In a fourth aspect of the present invention, a nonaqueous electrolytesecondary battery is arranged in any one of the first through thirdaspects to further include: another porous layer which is providedbetween (i) the nonaqueous electrolyte secondary battery separator and(ii) at least one of the positive electrode plate and the negativeelectrode plate.

In a fifth aspect of the present invention, a nonaqueous electrolytesecondary battery is arranged such that, in the fourth aspect of thepresent invention, the another porous layer contains at least one resinselected from the group consisting of a polyolefin, a(meth)acrylate-based resin, a fluorine-containing resin (excluding apolyvinylidene fluoride-based resin), a polyamide-based resin, apolyester-based resin, and a water-soluble polymer.

In a sixth aspect of the present invention, a nonaqueous electrolytesecondary battery is arranged such that, in the fifth aspect of thepresent invention, the polyamide-based resin is an aramid resin.

Advantageous Effects of Invention

An aspect of the present invention makes it possible to achieve anonaqueous electrolyte secondary battery that has an excellent chargecapacity characteristic after being subjected to high-rate discharge.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view schematically illustrating a measurement targetelectrode whose capacitance was to be measured in Examples of thepresent application.

FIG. 2 is a view schematically illustrating a probe electrode which wasused to measure the capacitance in Examples of the present application.

FIG. 3 is a view schematically illustrating (i) a device used in ascratch test in accordance with an embodiment of the present inventionand (ii) an operation of the device.

FIG. 4 is a graph which has been made based on results of a scratch testin accordance with an embodiment of the present invention and whichshows, as an example, a relationship between (a) a critical load and (b)a distance at which the critical load is reached.

DESCRIPTION OF EMBODIMENTS Embodiment 1

The following description will discuss an embodiment of the presentinvention. The present invention is, however, not limited to theembodiment below. The present invention is not limited to thearrangements described below, but may be altered in various ways by askilled person within the scope of the claims. Any embodiment based on aproper combination of technical means disclosed in different embodimentsis also encompassed in the technical scope of the present invention.Note that numerical expressions such as “A to B” herein mean “not lessthan A and not more than B” unless otherwise stated.

A nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention includes: a nonaqueous electrolytesecondary battery separator including a polyolefin porous film; a porouslayer containing a polyvinylidene fluoride-based resin (hereinafter alsoreferred to as a “PVDF-based resin”); a positive electrode plate havinga capacitance of not less than 1 nF and not more than 1000 nF permeasurement area of 900 mm²; and a negative electrode plate having acapacitance of not less than 4 nF and not more than 8500 nF permeasurement area of 900 mm², the polyolefin porous film having apiercing strength of not less than 26.0 gf/g/m², the piercing strengthbeing measured with respect to a weight per unit area of the polyolefinporous film, the polyolefin porous film having a value in a range of0.00 to 0.54, the value being represented by the following Expression(1):

|1−T/M|  (1)

where T represents a distance at which a critical load is reached in ascratch test in which the polyolefin porous film is moved in atransverse direction under a constant load of 0.1 N, and M represents adistance at which a critical load is reached in a scratch test in whichthe polyolefin porous film is moved in a machine direction under aconstant load of 0.1 N,

the porous layer being provided between the nonaqueous electrolytesecondary battery separator and at least one of the positive electrodeplate and the negative electrode plate, and the polyvinylidenefluoride-based resin containing an α-form polyvinylidene fluoride-basedresin in an amount of not less than 35.0 mol % with respect to 100 mol %of a total amount of the α-form polyvinylidene fluoride-based resin anda β-form polyvinylidene fluoride-based resin contained in thepolyvinylidene fluoride-based resin, where the amount of the α-formpolyvinylidene fluoride-based resin contained is calculated fromwaveform separation of (α/2) observed at around −78 ppm in a ¹⁹F-NMRspectrum obtained from the porous layer, and waveform separation of{(α/2)+β} observed at around −95 ppm in the ¹⁹F-MMR spectrum obtainedfrom the porous layer.

The term “measurement area” herein means an area of a part of ameasurement electrode (an upper (main) electrode or a probe electrode)of an LCR meter which part is in contact with a measurement target (apositive electrode plate or a negative electrode plate) in a case wherea capacitance is measured by a method for measuring a capacitance(described later). Therefore, a value of a capacitance per measurementarea of X mm² means a value obtained in a case where a capacitance ismeasured with use of an LCR meter while a measurement target and ameasurement electrode are in contact with each other so that a part ofthe measurement electrode which part is in contact with the measurementtarget has an area of X mm².

<Capacitance>

According to an embodiment of the present invention, a value of acapacitance of a positive electrode plate is a value measured by amethod for measuring a capacitance of an electrode plate (describedlater), that is, a value measured while a measurement electrode (probeelectrode) is in contact with a surface of the positive electrode plateon which surface a positive electrode active material layer is provided.The capacitance of the positive electrode plate mainly indicates apolarization state of the positive electrode active material layer ofthe positive electrode plate.

According to an embodiment of the present invention, a value of acapacitance of a negative electrode plate is a value measured by amethod for measuring a capacitance of an electrode plate (describedlater), that is, a value measured while a measurement electrode is incontact with a surface of the negative electrode plate on which surfacea negative electrode active material layer is provided. The capacitanceof the negative electrode plate mainly indicates a polarization state ofthe negative electrode active material layer of the negative electrodeplate.

In a case where the nonaqueous electrolyte secondary battery isdischarged, ions which are charge carriers are released from thenegative electrode plate. The ions thus released pass through thenonaqueous electrolyte secondary battery separator and then are takeninto the positive electrode plate. In this case, the ions are solvated,by an electrolyte solvent, in the negative electrode plate and on asurface of the negative electrode plate, and are desolvated in thepositive electrode plate and on a surface of the positive electrodeplate. Note that the ions are each Li⁺ in a case where, for example, thenonaqueous electrolyte secondary battery is a lithium-ion secondarybattery.

Thus, a degree to which the ions are solvated depends on thepolarization state of the negative electrode active material layer ofthe negative electrode plate. A degree to which the ions are desolvateddepends on the polarization state of the positive electrode activematerial layer of the positive electrode plate.

Therefore, the above-described solvation can be adequately promoted bycontrolling the capacitances of the negative electrode plate and thepositive electrode plate so that the capacitances fall within respectivesuitable ranges, i.e., by controlling the polarization states of thenegative electrode active material layer and the positive electrodeactive material layer so that the polarization states are suitable. This(i) allows enhancement of permeability to the ions which are chargecarriers and (ii) allows the nonaqueous electrolyte secondary battery tohave an enhanced discharge output characteristic especially in a casewhere a large electric current is discharged, at a rate of not less than20 C, from the nonaqueous electrolyte secondary battery. In view of theabove description, the negative electrode plate of the nonaqueouselectrolyte secondary battery in accordance with an embodiment of thepresent invention has a capacitance of not less than 4 nF and not morethan 8500 nF, preferably not less than 4 nF and not more than 3000 nF,and more preferably not less than 4 nF and not more than 2600 nF, permeasurement area of 900 mm². Note that the capacitance can have a lowerlimit that is not less than 100 nF, not less than 200 nF, or not lessthan 1000 nF.

Specifically, in a case where the negative electrode plate has acapacitance of less than 4 nF per measurement area of 900 mm²,polarizability of the negative electrode plate is so low that thenegative electrode plate hardly contributes to promotion of theabove-described solvation. Therefore, the nonaqueous electrolytesecondary battery which includes such a negative electrode plate doesnot have an enhanced output characteristic. Meanwhile, in a case wherethe negative electrode plate has a capacitance of more than 8500 nF permeasurement area of 900 mm², polarizability of the negative electrodeplate is so high that inner walls of voids in the negative electrodeplate have a too high affinity for the ions. This prevents the ions frommoving (being released) from the negative electrode plate. Therefore,the nonaqueous electrolyte secondary battery which includes such anegative electrode plate rather has a deteriorated outputcharacteristic.

In view of the above description, the positive electrode plate of thenonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention has a capacitance of not less than 1nF and not more than 1000 nF, preferably not less than 2 nF and not morethan 600 nF, and more preferably not less than 2 nF and not more than400 nF, per measurement area of 900 mm². Note that the capacitance canhave a lower limit that is not less than 3 nF.

Specifically, in a case where the positive electrode plate has acapacitance of less than 1 nF per measurement area of 900 mm²,polarizability of the positive electrode plate is so low that thepositive electrode plate hardly contributes to the above-describeddesolvation. Therefore, the nonaqueous electrolyte secondary batterywhich includes such a positive electrode plate does not have an enhancedoutput characteristic. Meanwhile, in a case where the positive electrodeplate has a capacitance of more than 1000 nF per measurement area of 900mm², polarizability of the positive electrode plate is so high that thedesolvation excessively progresses. Thus, the electrolyte solvent forthe ions to move inside the positive electrode plate is desolvated, andinner walls of voids in the positive electrode plate have a too highaffinity for the ions which have been desolvated. This prevents the ionsfrom moving inside the positive electrode plate. Therefore, thenonaqueous electrolyte secondary battery which includes such a positiveelectrode plate rather has a deteriorated output characteristic.

<Method for Adjusting Capacitance>

It is possible to control the capacitance of the positive electrodeplate by adjusting a surface area of the positive electrode activematerial layer, and it is possible to control the capacitance of thenegative electrode plate by adjusting a surface area of the negativeelectrode active material layer. Specifically, by, for example, rubbinga surface of each of the positive electrode active material layer andthe negative electrode active material layer with use of an abrasivepaper or the like, it is possible to increase the surface area of eachof the positive electrode active material layer and the negativeelectrode active material layer, and consequently to increase thecapacitance of each of the positive electrode plate and the negativeelectrode plate. Alternatively, it is possible to adjust the capacitanceof the positive electrode plate by adjusting a relative dielectricconstant of a material of which the positive electrode plate is made,and it is possible to control the capacitance of the negative electrodeplate by adjusting a relative dielectric constant of a material of whichthe negative electrode plate is made. The relative dielectric constantcan be adjusted by changing the shape of the voids in each of thepositive electrode plate and the negative electrode plate, a porosity ofeach of the positive electrode plate and the negative electrode plate,and distribution of the voids in each of the positive electrode plateand the negative electrode plate. The relative dielectric constant canbe alternatively controlled by adjusting the material of which each ofthe positive electrode plate and the negative electrode plate is made.

<Method for Measuring Capacitance of Electrode Plate>

According to an embodiment of the present invention, the capacitance ofthe electrode plate (positive electrode plate or negative electrodeplate) per measurement area of 900 mm² is measured with use of an LCRmeter, at a frequency of 300 KHz, and under conditions set as follows:CV: 0.010 V, SPEED: SLOW2, AVG: 8, CABLE: 1 m, OPEN: All, SHORT: AllDCBIAS 0.00 V.

Note that, according to the above measurement of the capacitance, thecapacitance of the electrode plate which has not been included in thenonaqueous electrolyte secondary battery is measured. Note that a valueof a capacitance is a unique value that is determined depending on, forexample, the shape (surface area) of a solid insulating material(electrode plate), a material of which the solid insulating material ismade, the shape of voids in the solid insulating material, a porosity ofthe solid insulating material, and distribution of the voids in thesolid insulating material. Therefore, the electrode plate which has beenincluded in the nonaqueous electrolyte secondary battery is equivalentin value of the capacitance to the electrode plate which has not beenincluded in the nonaqueous electrolyte secondary battery.

Note that the capacitance of each of the positive electrode plate andthe negative electrode plate can be measured after (i) the positiveelectrode plate and the negative electrode plate are included in thenonaqueous electrolyte secondary battery, (ii) the nonaqueouselectrolyte secondary battery is charged and discharged, and then (iii)the positive electrode plate and the negative electrode plate are takenout from the nonaqueous electrolyte secondary battery. Specifically, forexample, an electrode laminated body (a member for a nonaqueouselectrolyte secondary battery (hereinafter referred to as a “nonaqueouselectrolyte secondary battery member”)) is taken out from an exteriormember of the nonaqueous electrolyte secondary battery, and isdismantled so that one electrode plate (positive electrode plate ornegative electrode plate) is taken out. From the one electrode platethus taken out, a test piece is cut off which has a size similar to thatof the electrode plate which is to be subjected to the measurement inthe above-described method for measuring a capacitance of an electrodeplate. Subsequently, the test piece thus obtained is cleaned severaltimes (e.g., three times) in diethyl carbonate (DEC). The cleaning is astep of removing, for example, an electrolyte, a product ofdecomposition of the electrolyte, and a lithium salt, each adhering to asurface of the test piece (electrode plate), by (i) placing the testpiece in the DEC so as to clean the test piece and then (ii) repeating,several times (e.g., three times), a step of replacing the DEC with newDEC and cleaning the test piece in the new DEC. A resultant electrodeplate which has been cleaned is sufficiently dried and then is used as ameasurement target electrode. The exterior member of the nonaqueouselectrolyte secondary battery, from which exterior member the electrodelaminated body is to be taken out, can be of any kind, and the electrodelaminated body can have a laminated structure of any kind.

<Nonaqueous Electrolyte Secondary Battery Separator>

A nonaqueous electrolyte secondary battery separator of an embodiment ofthe present invention includes a polyolefin porous film. Note that inthe following description, the polyolefin porous film may also bereferred to as a “porous film”.

The porous film alone can be a nonaqueous electrolyte secondary batteryseparator. The porous film can also be a base material of a laminatedseparator for a nonaqueous electrolyte secondary battery (hereinafterreferred to as a “nonaqueous electrolyte secondary battery laminatedseparator”) (described later) in which a porous layer is disposed on theporous film. The porous film contains a polyolefin-based resin as a maincomponent and has therein many pores connected to one another. Thisallows a gas and a liquid to pass through the porous film from onesurface to the other.

The nonaqueous electrolyte secondary battery separator of an embodimentof the present invention can be arranged such that a porous layercontaining a polyvinylidene fluoride-based resin (described later) isdisposed on at least one of surfaces of the nonaqueous electrolytesecondary battery separator. In this case, a laminated body including(i) the nonaqueous electrolyte secondary battery separator and (ii) theporous layer which is disposed on at least one of the surfaces of thenonaqueous electrolyte secondary battery separator is herein referred toas a “nonaqueous electrolyte secondary battery laminated separator”. The“nonaqueous electrolyte secondary battery laminated separator” may behereinafter referred to as a “laminated separator”. The nonaqueouselectrolyte secondary battery separator of an embodiment of the presentinvention can further include other layer(s), different from thepolyolefin porous film, such as an adhesive layer, a heat-resistantlayer, and/or a protective layer.

(Polyolefin Porous Film)

The porous film contains polyolefin in an amount of not less than 50% byvolume, preferably not less than 90% by volume, and more preferably notless than 95% by volume, with respect to the entire porous film. Thepolyolefin more preferably contains a high molecular weight componenthaving a weight-average molecular weight of 5×10⁵ to 15×10⁶. Inparticular, the polyolefin which contains a high molecular weightcomponent having a weight-average molecular weight of not less than1,000,000 is more preferable because such polyolefin allows a nonaqueouselectrolyte secondary battery separator to have a higher strength.

Specific examples of the polyolefin, which is a thermoplastic resin,include a homopolymer and a copolymer each obtained by (co)polymerizinga monomer(s) such as ethylene, propylene, 1-butene, 4-methyl-1-pentene,and/or 1-hexene. Examples of the homopolymer include polyethylene,polypropylene, and polybutene. Examples of the copolymer include anethylene-propylene copolymer.

Among the above examples, polyethylene is more preferable. This isbecause polyethylene is capable of preventing (shutting down) a flow ofan excessively large electric current at a lower temperature. Examplesof the polyethylene include low-density polyethylene, high-densitypolyethylene, linear polyethylene (ethylene-α-olefin copolymer), andultra-high molecular weight polyethylene having a weight-averagemolecular weight of not less than 1,000,000. Of these polyethylenes,ultra-high molecular weight polyethylene having a weight-averagemolecular weight of not less than 1,000,000 is more preferable.

The porous film has a film thickness of preferably 4 μm to 40 μm, morepreferably 5 μm to 30 μm, and still more preferably 6 μm to 15 μm.

The porous film can have a weight per unit area which weight isdetermined as appropriate in view of the strength, the film thickness,the weight, and handleability of a nonaqueous electrolyte secondarybattery laminated separator including the porous film. Note, however,that the porous film has a weight per unit area of preferably 4 g/m² to20 g/m², more preferably 4 g/m² to 12 g/m², and still more preferably 5g/m² to 10 g/m², so as to allow the nonaqueous electrolyte secondarybattery which includes the nonaqueous electrolyte secondary batterylaminated separator including the porous film to have a higher weightenergy density and a higher volume energy density.

The porous film has a piercing strength of preferably not less than 26.0gf/g/m², and more preferably not less than 30.0 gf/g/m², the piercingstrength being measured with respect to a weight per unit area of theporous film. The porous film which has an excessively small piercingstrength, i.e., the porous film which has a piercing strength of lessthan 26.0 gf/g/m² is not preferable for the following reason.Specifically, when a nonaqueous electrolyte secondary battery separatorincluding such a porous film is used, the separator may be pierced bypositive electrode active material particles and negative electrodeactive material particles in a case where, for example, (i) anlaminating and winding operation to laminate and wind a positiveelectrode, a negative electrode, and the separator is carried out duringa battery assembling process, (ii) an operation to press a wound groupwhich has been subjected to the laminating and winding operation, or(iii) a pressure is externally applied to the battery. This may cause ashort circuit between the positive electrode and the negative electrode.

The porous film has an air permeability of preferably 30 sec/100 mL to500 sec/100 mL, and more preferably 50 sec/100 mL to 300 sec/100 mL, interms of Gurley values. The porous film which has an air permeabilityfalling within the above range makes it possible to achieve sufficiention permeability.

The porous film has a porosity of preferably 20% by volume to 80% byvolume, and more preferably 30% by volume to 75% by volume, so as to (i)retain a larger amount of electrolyte and (ii) obtain a function ofreliably preventing (shutting down) a flow of an excessively largeelectric current at a lower temperature. Further, in order to achievesufficient ion permeability and prevent particles from entering apositive electrode and/or a negative electrode of the nonaqueouselectrolyte secondary battery, the porous film has pores each having apore size of preferably not more than 0.3 μm, and more preferably notmore than 0.14 μm.

The porous film can be produced by, for example, (1) a method ofobtaining a porous film by adding a filler (pore forming agent) to aresin such as polyolefin, shaping the resin into a sheet, then removingthe filler with use of an appropriate solvent, and stretching the sheetfrom which the filler has been removed, or (2) a method of obtaining aporous film by adding a filler to a resin such as polyolefin, shapingthe resin into a sheet, then stretching the sheet, and removing thefiller from the sheet which has been stretched. That is, a resultantporous film normally contains no filler.

The porous film has a value in a range of 0.00 to 0.54, preferably 0.00to 0.50, and more preferably 0.00 to 0.45, the value being representedby the following Expression (1):

|1−T/M|  (1)

where T represents a distance at which a critical load is reached in ascratch test in which the polyolefin porous film is moved in atransverse direction under a constant load of 0.1 N, and M represents adistance at which a critical load is reached in a scratch test in whichthe polyolefin porous film is moved in a machine direction under aconstant load of 0.1 N,

The porous film also has a value in a range of preferably 0.00 to 0.54,more preferably 0.00 to 0.50, and still more preferably 0.00 to 0.45,the value being represented by the following Expression (2):

1−T/M   (2)

where T represents a distance at which a critical load is reached in ascratch test in which the porous film is moved in a TD under a constantload of 0.1 N, and M represents a distance at which a critical load isreached in a scratch test in which the porous film is moved in an MDunder a constant load of 0.1 N.

The values represented by the respective Expressions (1) and (2) areeach a value indicative of anisotropy of a distance at which a criticalload is reached (hereinafter referred to as a “critical load distance”)in a scratch test. The value which is closer to zero indicates that thecritical load distance is more isotropic.

FIG. 3 is a view schematically illustrating (i) a device used in ascratch test in accordance with an embodiment of the present inventionand (ii) an operation of the device. In FIG. 3, reference number 1indicates a diamond indenter, reference number 2 indicates a substrate,and reference number 3 indicates a porous film.

As illustrated in FIG. 3, a “scratch test” in accordance with anembodiment of the present invention is a test for measuring a stressthat occurs in a distance by which the porous film 3 is moved in ahorizontal direction while a surface layer of the porous film 3, whichis a measurement target, is subjected to compressive deformation in athicknesswise direction of the porous film 3 by applying a certain loadto the diamond indenter 1 (i.e. while the diamond indenter 1 is presseddown). Specifically, the scratch test is carried out by a methodincluding the following steps.

-   (1) The porous film 3, which is a measurement target, is cut into a    piece of 20 mm×60 mm. Then, the piece of the porous film 3 and a    substrate (glass preparation) 2 of 30 mm×70 mm are combined with use    of aqueous glue. Then, the piece and the substrate thus combined are    dried at a temperature of 25° C. for one whole day and night, so    that a test sample is prepared. Note that the piece of the porous    film 3 and the substrate (glass preparation) 2 are to be carefully    combined so that no air bubbles are made therebetween.-   (2) The test sample prepared in the step (1) is placed on a    microscratch testing device (manufactured by CSEM Instruments).    Then, while the diamond indenter 1 of the testing device is applying    a vertical load of 0.1 N to the test sample, a table of the testing    device is moved by a distance of 10 mm in a TD of the porous film 3    at a speed of 5 mm/min. During the movement of the table, a stress    (force of friction) that occurs between the diamond indenter 1 and    the test sample is measured.-   (3) A line graph is made which shows a relationship between    displacement of the stress measured in the step (2) and the distance    of the movement of the table. Then, based on the line graph, (i) a    critical load value in the TD and (ii) a distance in the TD at which    distance a critical load is reached are calculated as illustrated in    FIG. 4. FIG. 4 is a graph which has been made based on results of a    scratch test in accordance with an embodiment of the present    invention and which shows, as an example, a relationship between (a)    a critical load and (b) a distance at which the critical load is    reached.-   (4) The direction in which the table is moved is changed to an MD,    and the above steps (1) through (3) are repeatedly carried out so    that (i) a critical load value in the MD and (ii) a distance in the    MD at which distance a critical load is reached are calculated. Note    that the “TD” means a “transverse direction” and the “MD” means a    “machine direction”.

Note that any conditions and the like that are different from theconditions described above and under which to carry out the measurementin the scratch test are similar to those disclosed in JIS R 3255.

The scratch test is a test in which measurement and calculation iscarried out by modeling a mechanism for influence of expansion of anelectrode active material layer during charge and discharge (expansionof a negative electrode during charge, and expansion of a positiveelectrode during discharge) of a nonaqueous electrolyte secondarybattery, which includes a nonaqueous electrolyte secondary batteryseparator including a porous film serving as a measurement target, on(i) a first surface layer of the porous film, which first surface layerfaces the expanded electrode, and (ii) a second surface layer of theporous film, which second surface layer is opposite the first surfacelayer.

Note here that the expansion and shrinkage of the electrode activematerial layer during charge and discharge of the nonaqueous electrolytesecondary battery causes the first surface layer of the porous film,which first surface layer faces the expanded electrode, to be deformed(subjected to compressive deformation) in a thicknesswise direction ofthe porous film by the expanded electrode active material layer. Inaddition, the expansion of the electrode active material layer in ahorizontal direction causes a shearing stress to occur in a surface-wisedirection of the porous film. Furthermore, the shearing stress istransmitted, via the porous film, to an interface between (a) the secondsurface layer of the porous film, which second surface layer is oppositethe first surface layer, which faces the expanded electrode, and (b) theelectrode.

Therefore, a critical load distance calculated by the scratch testserves as (a) an indicator of whether a surface layer of the porous filmwhich surface layer faces the electrode is easily plastically-deformedand (b) an indicator of how easily a shearing stress is transmitted to asurface of the porous film which surface is opposite a measured surface(surface facing the electrode) of the porous film. A long critical loaddistance indicates that (a′) a surface layer part of a porous film to bemeasured is not easily plastically-deformed and (b′) a shearing stressis less easily transmitted to a surface, opposite a measured surface, ofthe porous film to be measured.

In view of the above description, the porous film which has a valuebeyond 0.54, the value being represented by the Expression (1) showsthat the porous film has a large anisotropy between the critical loaddistance in the TD and the critical load distance in the MD. In anonaqueous electrolyte secondary battery which includes either of anonaqueous electrolyte secondary battery separator and a nonaqueouselectrolyte secondary battery laminated separator each including theporous film which has such a large anisotropy, a wrinkle and a gap occurat an interface between (a) the nonaqueous electrolyte secondary batteryseparator or the nonaqueous electrolyte secondary battery laminatedseparator and (b) the electrode predominantly in a certain direction.The occurrence of the wrinkle and the gap is due to (i) a differencebetween the TD and the MD in size of plastic deformation, caused bybattery charge and discharge, of the surface layer of the porous filmand (ii) a difference between the TD and the MD in easiness oftransmission of a surface stress to a surface of the porous film whichsurface is opposite a surface of the porous film which surface faces theexpanded electrode.

Since the nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention has a value in a range of 0.00 to0.54, the value being represented by the Expression (1), the criticalload distance is isotropic. This makes it possible to restrain a porestructure of the porous film from being deformed due to charge anddischarge of the battery. As a result, it is possible to restrain adeterioration in performance of the battery.

Note that a physical property value of the porous film on which a porouslayer and/or another layer are/is disposed can be measured after theporous layer and/or the another layer are/is removed from a laminatedbody including the porous film and the porous layer and/or the anotherlayer. The porous layer and/or the another layer can be removed from thelaminated body by, for example, a method of dissolving a resin of theporous layer and/or the another layer with use of a solvent such asN-methylpyrrolidone or acetone so as to remove the resin.

The following description will discuss a nonaqueous electrolytesecondary battery which is an aspect of a laminated secondary batteryincluding electrode plates and a nonaqueous electrolyte secondarybattery separator and which is arranged such that a laminated bodyincluding the electrode plates and the nonaqueous electrolyte secondarybattery separator is wound. Note that a laminated secondary batteryherein refers to a nonaqueous electrolyte secondary battery having astructure in which electrodes and a nonaqueous electrolyte secondarybattery separator are disposed.

In the nonaqueous electrolyte secondary battery which is arranged suchthat the laminated body is wound, the nonaqueous electrolyte secondarybattery separator is wound while a tensile force is being appliedthereto in a machine direction. This causes an increase in smoothness inthe machine direction of a porous film, and causes an internal stress tobe inwardly applied in a transverse direction of the porous film.Therefore, according to the nonaqueous electrolyte secondary batterywhich is arranged such that the laminated body is wound, (i) a criticalload distance in the machine direction during actual operation of thenonaqueous electrolyte secondary battery is longer than a critical loaddistance in a machine direction which critical load distance iscalculated in the scratch test, and (ii) a critical load distance in thetransverse direction during actual operation of the nonaqueouselectrolyte secondary battery is shorter than a critical load distancein a transverse direction which critical load distance is calculated inthe scratch test.

Therefore, in a case where a critical load distance in the transversedirection and a critical load distance in the machine direction aresimilar (i.e. highly isotropic), specifically, in a case where a porousfilm having a value of not less than −0.54 and less than 0.00, the valuebeing represented by the Expression (2), is used as a separator or as amember of a separator in a nonaqueous electrolyte secondary batterywhich is arranged such that a laminated body is wound, the critical loaddistance in the machine direction increases, and the critical loaddistance in the transverse direction decreases.

Therefore, in actual operation of the nonaqueous electrolyte secondarybattery, a wrinkle and a gap occur at an interface between (a) theporous film and (b) the electrode predominantly in the transversedirection. The occurrence of the wrinkle and the gap is due to (i) adifference between the transverse direction and the machine direction insize of plastic deformation of a surface layer of the porous film and(ii) a difference between the transverse direction and the machinedirection in easiness of transmission of a surface stress to a surfaceof the porous film which surface is opposite a surface of the porousfilm which surface faces the expanded electrode. This causes a decreasein surface-wise uniformity in distance between the electrodes.

Meanwhile, in a case where the nonaqueous electrolyte secondary batterywhich is arranged such that the laminated body is wound has highlyanisotropic critical load distances in the transverse direction and inthe machine direction, specifically, in a case where the valuerepresented by the Expression (1) is beyond 0.54, for a reason similarto the reason described earlier, more wrinkles and more gaps occur at aninterface between (a) the porous film and (b) the electrode in adirection in which a critical load distance is longer. The occurrence ofthe wrinkles and the gaps is due to (i) a difference between thetransverse direction and the machine direction in size of plasticdeformation of a surface layer of the porous film and (ii) a differencebetween the transverse direction and the machine direction in easinessof transmission of a surface stress to a surface of the porous filmwhich surface is opposite a surface of the porous film which surfacefaces the expanded electrode. This causes a reduction in a dischargerate characteristic maintenance rate of the nonaqueous electrolytesecondary battery which has been subjected to a discharge cycle.Therefore, the value represented by the Expression (2) is preferably ina range of 0.00 to 0.54. This is because the porous film which has sucha value can be suitably used for the nonaqueous electrolyte secondarybattery which is arranged such that the laminated body is wound.

Note that a critical load distance in a transverse direction and acritical load distance in a machine direction are considered to begreatly affected by the following structure factors of a porous film.

-   (i) How resin polymers are oriented in the machine direction of the    porous film-   (ii) How resin polymers are oriented in the transverse direction of    the porous film-   (iii) How the resin polymers which are oriented in the machine    direction and the resin polymers which are oriented in the    transverse direction are in contact with each other in a    thicknesswise direction of the porous film

Therefore, the value represented by the Expression (1) and the valuerepresented by the Expression (2) can be controlled by, for example,controlling the above structure factors (i) through (iii) by adjustingthe following production conditions under which a porous film productionmethod (described later) is carried out.

-   (1) Circumferential velocity [m/min] of rolling mill roll-   (2) Ratio of stretch temperature to stretch ratio [° C./%]

Specifically, the circumferential velocity of the rolling mill roll andthe ratio of the stretch temperature during stretching to the stretchratio are adjusted so that the circumferential velocity of the rollingmill roll, the stretch temperature during stretching, and the stretchratio satisfy a relationship represented by Expression (3) below,provided that production of the porous film is not hindered. This allowseach of the value represented by the Expression (1) and the valuerepresented by the Expression (2) to be controlled in a range of 0.00 to0.54.

Y≥−2.3×X+22.2   (3)

where (i) X represents the circumferential velocity of the rolling millroll and (ii) Y represents the ratio of the stretch temperature duringstretching to the stretch ratio in the transverse direction.

Meanwhile, in a case where the circumferential velocity of the rollingmill roll and the ratio of the stretch temperature during stretching tothe stretch ratio are set so as to fall outside the range satisfying therelationship represented by the Expression (3), (i) the orientation ofthe resin polymers in the machine direction of the porous film or theorientation of the resin polymers in the transverse direction of theporous film is promoted and/or (ii) connectivity, in the thicknesswisedirection of the porous film, of the resin polymers which are orientedin the machine direction or of the resin polymers which are oriented inthe transverse direction is promoted. This causes the anisotropy of theporous film which anisotropy is represented by the Expression (1) to belarge, so that the value represented by the Expression (1) cannot becontrolled in a range of 0.00 to 0.54. For example, in a case where thecircumferential velocity of the rolling mill roll is adjusted to 2.5m/min and the ratio of the stretch temperature to the stretch ratio isadjusted to less than 16.5° C./%, (i) the orientation of the resinpolymers in the transverse direction of the porous film is promoted and(ii) connectivity, in the thicknesswise direction of the porous film, ofthe resin polymers which are oriented in the transverse direction ispromoted. This causes a critical load distance in the transversedirection to be short, so that the anisotropy represented by theExpression (1) is more than 0.54.

The stretch temperature is preferably 90° C. to 120° C., and morepreferably 100° C. to 110° C. The stretch ratio is preferably 600% to800%, and more preferably 620% to 700%.

The structure factor (i) can be controlled mainly by the productioncondition (1). The structure factor (ii) can be controlled mainly by theproduction condition (2). The structure factor (iii) can be controlledmainly by a combination of the production conditions (1) and (2).

In a case where the nonaqueous electrolyte secondary battery laminatedseparator in accordance with an embodiment of the present invention isproduced, i.e., in a case where the porous layer containing apolyvinylidene fluoride-based resin (described later) is disposed on theporous film, the porous film is more preferably subjected to ahydrophilization treatment before the porous layer is formed, that is,before the porous film is coated with a coating solution (describedlater).

The porous film which has been subjected to the hydrophilizationtreatment is more coatable with a coating solution. This allows a moreuniform porous layer to be formed. The hydrophilization treatment iseffective in a case where water accounts for a high proportion of asolvent (dispersion medium) contained in the coating solution. Specificexamples of the hydrophilization treatment include publicly knowntreatments such as (i) a chemical treatment carried out with use of anacid, an alkali, or the like, (ii) a corona treatment, and (iii) aplasma treatment. Of these hydrophilization treatments, the coronatreatment is more preferable. This is because the corona treatmentallows a porous film to be hydrophilized in a relatively short period oftime and causes only a surface and its vicinity of the porous film to behydrophilized, so that the inside of the porous film remains unchangedin quality.

(Porous Layer)

A porous layer is provided between the nonaqueous electrolyte secondarybattery separator and at least one of the positive electrode plate andthe negative electrode plate so as to serve as a member of a nonaqueouselectrolyte secondary battery. The porous layer can be disposed on onesurface or both surfaces of the nonaqueous electrolyte secondary batteryseparator. Alternatively, the porous layer can be disposed on an activematerial layer of at least one of the positive electrode plate and thenegative electrode plate. Alternatively, the porous layer can beprovided between the nonaqueous electrolyte secondary battery separatorand at least one of the positive electrode plate and the negativeelectrode plate so as to be in contact with the nonaqueous electrolytesecondary battery separator and with the at least one of the positiveelectrode plate and the negative electrode plate. The number of porouslayer(s) which is/are provided between the nonaqueous electrolytesecondary battery separator and at least one of the positive electrodeplate and the negative electrode plate can be one, or two or more.

A porous layer is preferably an insulating porous layer containing aresin.

It is preferable that a resin which can be contained in the porous layerbe insoluble in an electrolyte of a battery and be electrochemicallystable when the battery is in normal use. In a case where the porouslayer is disposed on one surface of the porous film, the porous layer isdisposed preferably on a surface of the porous film which surface facesthe positive electrode plate of the nonaqueous electrolyte secondarybattery, and more preferably on a surface of the porous film whichsurface is in contact with the positive electrode plate.

A porous layer of an embodiment of the present invention contains aPVDF-based resin, the PVDF-based resin containing a PVDF-based resinhaving crystal form a (hereinafter, referred to as an α-form PVDF-basedresin) in an amount of not less than 35.0 mol % with respect to 100 mol% of a total amount of the α-form PVDF-based resin and a PVDF-basedresin having crystal form β (hereinafter, referred to as a β-formPVDF-based resin), the α-form PVDF-based resin and the β-form PVDF-basedresin each being contained in the PVDF-based resin.

Note here that the amount of the α-form PVDF-based resin contained iscalculated from waveform separation of (α/2) observed at around −78 ppmin a ¹⁹F-NMR spectrum obtained from the porous layer, and waveformseparation of {(α/2)+β} observed at around −95 ppm in the ¹⁹F-MMRspectrum obtained from the porous layer.

The porous layer has a structure in which many pores, connected to oneanother, are provided, so that the porous layer is a layer through whicha gas or a liquid can pass from one surface to the other. Further, in acase where the porous layer of an embodiment of the present invention isused as a member of a nonaqueous electrolyte secondary battery laminatedseparator, the porous layer can be a layer which, while serving as anoutermost layer of the nonaqueous electrolyte secondary batterylaminated separator, adheres to an electrode.

Examples of the PVDF-based resin include homopolymers of vinylidenefluoride; copolymers of vinylidene fluoride and other monomer(s)copolymerizable with vinylidene fluoride; and mixtures of the abovepolymers. Examples of a monomer polymerizable with vinylidene fluorideinclude hexafluoropropylene, tetrafluoroethylene, trifluoroethylene,trichloroethylene, and vinyl fluoride. These monomers can be used in onekind or in combination of two or more kinds. The PVDF-based resin can besynthesized through emulsion polymerization or suspensionpolymerization.

The PVDF-based resin contains vinylidene fluoride in an amount ofnormally not less than 85 mol %, preferably not less than 90 mol %, morepreferably not less than 95 mol %, and still more preferably not lessthan 98 mol %. The PVDF-based resin which contains vinylidene fluoridein an amount of not less than 85 mol % is more likely to allow theporous layer to have a mechanical strength against pressure and a heatresistance against heat during battery production.

In another aspect, the porous layer preferably contains two kinds ofPVDF-based resins (that is, a first resin and a second resin below) thatdiffer from each other in, for example, amount of hexafluoropropylenecontained.

The first resin is a vinylidene fluoride-hexafluoropropylene copolymercontaining hexafluoropropylene in an amount of more than 0 mol % and notmore than 1.5 mol % or (ii) a vinylidene fluoride homopolymer.

The second resin is a vinylidene fluoride-hexafluoropropylene copolymercontaining hexafluoropropylene in an amount of more than 1.5 mol %.

The porous layer which contains the two kinds of PVDF-based resinsadheres better to an electrode as compared with the porous layer whichdoes not contain one of the two kinds of PVDF-based resins. Further, ascompared with the porous layer which does not contain one of the twokinds of PVDF-based resins, the porous layer which contains the twokinds of PVDF-based resins adheres better to another layer (e.g., aporous film layer) of the nonaqueous electrolyte secondary batteryseparator, and consequently a higher peel force is required to peel theporous layer and the another layer from each other. The first resin andthe second resin preferably have therebetween a mass ratio of 15:85 to85:15.

The PVDF-based resin preferably has a weight-average molecular weight of200,000 to 3,000,000, more preferably 200,000 to 2,000,000, and stillmore preferably 500,000 to 1,500,000. The PVDF-based resin which has aweight-average molecular weight of not less than 200,000 tends to allowthe porous layer and the electrode to adhere well to each other.Meanwhile, the PVDF-based resin which has a weight-average molecularweight of not more than 3,000,000 tends to be easily formable.

The porous layer of an embodiment of the present invention can contain aresin which is different from the PVDF-based resin and is exemplified bystyrene-butadiene copolymers; homopolymers or copolymers of vinylnitriles such as acrylonitrile and methacrylonitrile; and polyetherssuch as polyethylene oxide and polypropylene oxide.

The porous layer of an embodiment of the present invention can contain afiller. The filler can be an inorganic filler (e.g., metal oxide fineparticles), an organic filler, or the like. The porous layer containsthe filler in an amount of preferably not less than 1% by mass and notmore than 99% by mass, and more preferably not less than 10% by mass andnot more than 98% by mass, with respect to a total amount of thePVDF-based resin and the filler. The amount of the filler contained inthe porous layer has a lower limit that can be not less than 50% bymass, not less than 70% by mass, or not less than 90% by mass. Theorganic filler can be a conventionally publicly known organic filler,and the inorganic filler can be a conventionally publicly knowninorganic filler.

In order to achieve (i) adhesiveness of the porous layer to an electrodeand (ii) a high energy density, the porous layer of an embodiment of thepresent invention has an average thickness of preferably 0.5 μm to 10 μm(per layer), and more preferably 1 μm to 5 μm (per layer).

The porous layer preferably has a thickness of not less than 0.5 μm (perlayer). This is because the porous layer which has such a thickness (i)makes it is possible to sufficiently restrain an internal short circuitthat might occur due to, for example, breakage of the nonaqueouselectrolyte secondary battery and (ii) allows the porous layer to retainan electrolyte in an adequate amount.

Meanwhile, the porous layer which has a thickness of more than 10 μm(per layer) causes an increase in resistance to lithium ion permeationin the nonaqueous electrolyte secondary battery. Thus, the nonaqueouselectrolyte secondary battery which is repeatedly subjected to chargeand discharge cycles deteriorates in positive electrode and alsodeteriorates in rate characteristic and cycle characteristic. Further,such a porous layer makes a distance between the positive electrode andthe negative electrode greater. This causes the nonaqueous electrolytesecondary battery to have a lower internal volume efficiency.

The porous layer of an embodiment of the present invention is preferablyprovided between the nonaqueous electrolyte secondary battery separatorand the positive electrode active material layer of the positiveelectrode plate. Physical properties of the porous layer which aredescribed below at least refer to physical properties of the porouslayer which serves as a member of the nonaqueous electrolyte secondarybattery and which is provided between the nonaqueous electrolytesecondary battery separator and the positive electrode active materialof the positive electrode plate.

The porous layer can have a weight per unit area (per layer) whichweight is appropriately determined in view of the strength, the filmthickness, the weight, and handleability of the porous layer. A coatingamount (weight per unit area) of the porous layer is preferably 0.5 g/m²to 20 g/m² per layer and more preferably 0.5 g/m² to 10 g/m² per layer.

The porous layer which has a weight per unit area which weight fallswithin the above numerical range allows the nonaqueous electrolytesecondary battery which includes the porous layer to have a higherweight energy density and a higher volume energy density. The porouslayer which has a weight per unit area which weight is more than theupper limit of the above range makes the nonaqueous electrolytesecondary battery heavy.

In order to achieve sufficient ion permeability, the porous layer has aporosity of preferably 20% by volume to 90% by volume, and morepreferably 30% by volume to 80% by volume. The porous layer has poreswhose pore size is preferably not more than 1.0 μm, and more preferablynot more than 0.5 μm. The pores which have a pore size falling withinthe above range allows the nonaqueous electrolyte secondary batterywhich includes the nonaqueous electrolyte secondary battery laminatedseparator including the porous layer to achieve sufficient ionpermeability.

The nonaqueous electrolyte secondary battery laminated separator has anair permeability of preferably 30 sec/100 mL to 1000 sec/100 mL, andmore preferably 50 sec/100 mL to 800 sec/100 mL, in terms of Gurleyvalues. The nonaqueous electrolyte secondary battery laminated separatorwhich has an air permeability falling within the above range allows thenonaqueous electrolyte secondary battery to achieve sufficient ionpermeability.

The air permeability which is less than the lower limit of the aboverange means that the nonaqueous electrolyte secondary battery laminatedseparator has a high porosity and thus has a coarse laminated structure.This may cause the nonaqueous electrolyte secondary battery laminatedseparator to have a lower strength and consequently to be insufficientparticularly in shape stability at a high temperature. Meanwhile, theair permeability which is more than the upper limit of the above rangemay prevent the nonaqueous electrolyte secondary battery laminatedseparator from achieving sufficient ion permeability and consequentlydegrade battery characteristics of the nonaqueous electrolyte secondarybattery.

(Crystal Forms of PVDF-Based Resin)

The PVDF-based resin contained in the porous layer which is used for anembodiment of the present invention contains an α-form PVDF-based resinin an amount of not less than 35.0 mol %, preferably not less than 37.0mol %, more preferably not less than 40.0 mol %, and still morepreferably not less than 44.0 mol %, with respect to 100 mol % of atotal amount of the α-form PVDF-based resin and a β-form PVDF-basedresin contained in the PVDF-based resin. Further, the amount of theα-form PVDF-based resin is preferably not more than 90.0 mol %. Theporous layer which contains the α-form PVDF-based resin in an amountfalling within the above range is suitably used as a member of anonaqueous electrolyte secondary battery that excels in retention of acharge capacity after being discharged at a high rate, in particular, amember of a nonaqueous electrolyte secondary battery laminated separatoror of a nonaqueous electrolyte secondary battery electrode.

The nonaqueous electrolyte secondary battery which is being charged anddischarged generates heat due to internal resistance thereof. A largerelectric current, that is, a higher rate condition leads to generationof a larger amount of heat. The α-form PVDF-based resin contained in thePVDF-based resin has a higher melting point than the β-form PVDF-basedresin contained in the PVDF-based resin, and is less likely to beplastically deformed by heat. Furthermore, the β-form PVDF-based resin,which has a structure in which F atoms are aligned in one direction, isknown to be more polarized than the α-form PVDF-based resin. Accordingto the porous layer of an embodiment of the present invention, in a casewhere the PVDF-based resin of the porous layer contains the α-formPVDF-based resin in an amount that is not less than a certain amount, itis possible to not only reduce (i) deformation of an internal structureof the porous layer, (ii) clogging of voids in the porous layer, and/or(iii) the like due to deformation of the PVDF-based resin whichdeformation is caused by generation of heat during charge and dischargeof the nonaqueous electrolyte secondary battery, in particular, duringoperation of the nonaqueous electrolyte secondary battery under a highrate condition, but also avoid uneven distribution of Li ions due tointeraction between the Li ions and the PVDF-based resin. As a result,it is possible to restrain a deterioration in performance of thebattery.

The α-form PVDF-based resin is arranged such that the PVDF-based resinis made of a polymer containing a PVDF skeleton. The PVDF skeleton has aconformation in which there are two or more consecutive chains of asteric structure in which molecular chains include a main-chain carbonatom bonded to a fluorine atom (or a hydrogen atom) adjacent to twocarbon atoms one of which is bonded to a hydrogen atom (or a fluorineatom) having a trans position and the other (opposite) one of which isbonded to a hydrogen atom (or a fluorine atom) having a gauche position(positioned at an angle of 60°), the conformation being the followingconformation:

(TGTG-type conformation)   [Math. 1]

and a molecular chain having the following type:

TGTG  [Math. 2]

wherein respective dipole moments of C—F₂ and C—H₂ bonds each have acomponent perpendicular to the molecular chain and a component parallelto the molecular chain.

The α-form PVDF-based resin has characteristic peaks at around −95 ppmand around −78 ppm in a ¹⁹F-NMR spectrum thereof.

The β-form PVDF-based resin is arranged such that the PVDF-based resinis made of a polymer containing a PVDF skeleton. The PVDF skeleton has aconformation in which molecular chains including a main-chain carbonatom adjacent to two carbon atoms bonded to a fluorine atom and ahydrogen atom, respectively, each having a trans configuration (TT-typeconformation), that is, the fluorine atom and the hydrogen atom whichare bonded to the respective two carbon atoms are positioned oppositelyat an angle of 180° as viewed in the direction of the carbon-carbonbond.

The β-form PVDF-based resin can be arranged such that the PVDF skeletoncontained in the polymer of the PVDF-based resin has a TT-typeconformation in its entirety. The β-form PVDF-based resin canalternatively be arranged such that the PVDF skeleton partially has theTT-type conformation and has a molecular chain of the TT-typeconformation in at least four consecutive PVDF monomeric units. In anyof the above cases, (i) the carbon-carbon bond, in which the TT-typeconformation constitutes a TT-type main chain, has a planar zigzagstructure, and (ii) the respective dipole moments of the C—F₂ and C—H₂bonds each have a component perpendicular to the molecular chain.

The β-form PVDF-based resin has a characteristic peak at around −95 ppmin a ¹⁹F-NMR spectrum thereof.

(Method of Calculating Respective Percentages of α-Form PVDF-Based Resinand β-Form PVDF-Based Resin Each Contained in PVDF-Based Resin)

Respective percentages of the α-form PVDF-based resin and the β-formPVDF-based resin, each contained in the porous layer of an embodiment ofthe present invention, with respect to 100 mol % of the total amount ofthe α-form PVDF-based resin and the β-form PVDF-based resin can becalculated from the a ¹⁹F-NMR spectrum obtained from the porous layer.Specifically, the percentages of the α-form PVDF-based resin and theβ-form PVDF-based resin can be calculated by, for example, the followingmethod.

-   (1) From a porous layer containing a PVDF-based resin, a ¹⁹F-NMR    spectrum is obtained under the following conditions.

Measurement Conditions

Measurement device: AVANCE400 manufactured by Bruker Biospin

Measurement method: Single-pulse method

Observed nucleus: ¹⁹F

Spectral bandwidth: 100 kHz

Pulse width: 3.0 s (90° pulse)

Pulse repetition time: 5.0 s

Reference material: C₆F₆ (external reference: −163.0 ppm)

Temperature: 22° C.

Sample rotation frequency: 25 kHz

-   (2) An integral value of a peak at around −78 ppm in the ¹⁹F-NMR    spectrum obtained in (1) is calculated and is regarded as an α/2    amount.-   (3) As in the case of (2), an integral value of a peak at around −95    ppm in the ¹⁹F-NMR spectrum obtained in (1) is calculated and is    regarded as an {(α/2)+β} amount.-   (4) A percentage of the α-form PVDF-based resin contained in the    porous layer (this percentage is hereinafter also referred to as an    α ratio) with respect to 100 mol % of a total amount of the α-form    PVDF-based resin and the β-form PVDF-based resin is calculated, from    the integral values obtained in (2) and (3), based on the following    equation (1):

α ratio(mol %)=[(integral value at around −78 ppm)×2/{(integral value ataround −95 ppm)+(integral value at around −78 ppm)}]×100   (1)

-   (5) A percentage of the β-form PVDF-based resin contained in the    porous layer (this percentage is hereinafter also referred to as a β    ratio) with respect to 100 mol % of the total amount of the α-form    PVDF-based resin and the β-form PVDF-based resin is calculated, from    a value of the α ratio obtained in (4), based on the following    equation (2):

β ratio(mol %)=100(mol %)−α ratio(mol %)   (2)

(Method for Producing Porous Layer and Nonaqueous Electrolyte SecondaryBattery Laminated Separator)

The porous layer of an embodiment of the present invention and anonaqueous electrolyte secondary battery laminated separator each can beproduced by a method that is not limited to any particular method butcan be any of various methods.

For example, a porous layer containing a PVDF-based resin and optionallya filler is formed, through one of the steps (1) through (3) below, on asurface of a porous film to serve as a base material. The steps (2) and(3) each further involve drying a deposited porous layer so as to removea solvent. Note that a coating solution that is used to produce theporous layer containing a filler and is used in each of the steps (1)through (3) is preferably a coating solution in which the filler isdispersed and in which the PVDF-based resin is dissolved.

The coating solution which is used in the method for producing theporous layer of an embodiment of the present invention can be preparednormally by (i) dissolving, in a solvent, a resin contained in theporous layer and (ii) dispersing, in a resultant solution, a fillercontained in the porous layer.

(1) Step of forming a porous layer by (i) coating a porous film with acoating solution containing a PVDF-based resin and optionally a fillerof each of which the porous layer is formed, and then (ii) drying thecoating solution so as to remove a solvent (dispersion medium) containedin the coating solution.

(2) Step of depositing a porous layer by (i) coating a surface of theporous film with the coating solution mentioned in the step (1), andthen (ii) immersing the porous film in a deposition solvent, which is apoor solvent with respect to the PVDF-based resin.

(3) Step of depositing a porous layer by (i) coating a surface of theporous film with the coating solution mentioned in the step (1), andthen (ii) making the coating solution acidic with use of a low-boilingorganic acid.

Examples of the solvent (dispersion medium) contained in the coatingsolution include N-methylpyrrolidone, N,N-dimethylacetamide,N,N-dimethylformamide, acetone, and water.

The deposition solvent is preferably isopropyl alcohol or t-butylalcohol, for example.

In the step (3), the low-boiling organic acid can be, for example,paratoluene sulfonic acid or acetic acid.

Note that the base material can be not only a porous film but also anyof a film different from the porous film, a positive electrode plate,and a negative electrode plate.

The coating solution can appropriately contain an additive(s) such as adispersing agent, a plasticizing agent, a surface active agent, and a pHadjusting agent as a component(s) different from the resin and thefiller.

The coating solution can be applied to the porous film by aconventionally publicly known method that is specifically exemplified bya gravure coater method, a dip coater method, a bar coater method, and adie coater method.

(Method for Controlling Crystal Forms of PVDF-Based Resin)

Crystal forms of the PVDF-based resin contained in the porous layer ofan embodiment of the present invention can be controlled by adjustingdrying conditions under which to carry out the above-described method(e.g., the drying temperature, and the air velocity and direction duringdrying), and/or the deposition temperature at which a porous layercontaining a PVDF-based resin is deposited with use of a depositionsolvent or a low-boiling organic acid.

The drying conditions and the deposition temperature, which are adjustedso that the PVDF-based resin contains an α-form PVDF-based resin in anamount of not less than 35.0 mol % with respect to 100 mol % of a totalamount of the α-form PVDF-based resin and a β-form PVDF-based resincontained in the PVDF-based resin, can be changed as appropriate bychanging, for example, the method for producing the porous layer, a kindof solvent (dispersion medium) to be used, a kind of deposition solventto be used, and/or a kind of low-boiling organic acid to be used.

In a case where the coating solution is simply dried as in the step (1),the drying conditions can be changed as appropriate by adjusting, forexample, the amount of the solvent in the coating solution, theconcentration of the PVDF-based resin in the coating solution, theamount of the filler (if contained), and/or the amount of the coatingsolution applied.

In a case where the porous layer is formed through the step (1), it ispreferable that the drying temperature be 30° C. to 100° C., that hotair blow, during drying, perpendicularly to a porous base material or anelectrode sheet to which the coating solution has been applied, and thatthe hot air blow at a velocity of 0.1 m/s to 40 m/s.

Specifically, in a case where a coating solution to be applied containsN-methyl-2-pyrrolidone as the solvent for dissolving the PVDF-basedresin, 1.0% by mass of the PVDF-based resin, and 9.0% by mass of aluminaas the inorganic filler, the drying conditions are preferably adjustedso that (i) the drying temperature is 40° C. to 100° C., (ii) hot airblows, during drying, perpendicularly to the porous base material or theelectrode sheet to which the coating solution has been applied, and(iii) the hot air blows at a velocity of 0.4 m/s to 40 m/s.

In a case where the porous layer is formed through the step (2), it ispreferable that the deposition temperature be −25° C. to 60° C. and thatthe drying temperature be 20° C. to 100° C. Specifically, in a casewhere the porous layer is formed through the step (2) with use ofN-methylpyrrolidone as the solvent for dissolving the PVDF-based resinand isopropyl alcohol as the deposition solvent, it is preferable thatthe deposition temperature be −10° C. to 40° C. and that the dryingtemperature be 30° C. to 80° C.

(Another Porous Layer)

The nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention can include another porous layer inaddition to (i) the porous film and (ii) the porous layer containing thePVDF-based resin. The another porous layer need only be provided between(i) the nonaqueous electrolyte secondary battery separator and (ii) atleast one of the positive electrode plate and the negative electrodeplate. The porous layer and the another porous layer can be provided inany order with respect to the nonaqueous electrolyte secondary batteryseparator. In a preferable arrangement, the porous film, the anotherporous layer, and the porous layer containing the PVDF-based resin aredisposed in this order. In other words, the another porous layer isprovided between the porous film and the porous layer containing thePVDF-based resin. In another preferable arrangement, the another porouslayer and the porous layer containing the PVDF-based resin are providedin this order on both surfaces of the porous film.

Further, the another porous layer of an embodiment of the presentinvention can contain a resin which is exemplified by polyolefins;(meth)acrylate-based resins; fluorine-containing resins (excludingpolyvinylidene fluoride-based resins); polyamide-based resins;polyimide-based resins; polyester-based resins; rubbers; resins eachhaving a melting point or a glass transition temperature of not lowerthan 180° C.; water-soluble polymers; and polycarbonate, polyacetal, andpolyether ether ketone.

Of the above resins, polyolefins, (meth)acrylate-based resins,polyamide-based resins, polyester-based resins, and water-solublepolymers are preferable.

Preferable examples of the polyolefins include polyethylene,polypropylene, polybutene, and an ethylene-propylene copolymer.

Examples of the fluorine-containing resins includepolytetrafluoroethylene, a vinylidene fluoride-hexafluoropropylenecopolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, atetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, a vinylidenefluoride-tetrafluoroethylene copolymer, a vinylidenefluoride-trifluoroethylene copolymer, a vinylidenefluoride-trichloroethylene copolymer, a vinylidene fluoride-vinylfluoride copolymer, a vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene copolymer, and anethylene-tetrafluoroethylene copolymer. Particular examples of thefluorine-containing resins include fluorine-containing rubber having aglass transition temperature of not higher than 23° C.

Preferable examples of the polyamide-based resins include aramid resinssuch as aromatic polyamides and wholly aromatic polyamides.

Specific examples of the aramid resins include poly(paraphenyleneterephthalamide), poly(metaphenylene isophthalamide),poly(parabenzamide), poly(metabenzamide), poly(4,4′-benzanilideterephthalamide), poly(paraphenylene-4,4′-biphenylene dicarboxylic acidamide), poly(metaphenylene-4,4′-biphenylene dicarboxylic acid amide),poly(paraphenylene-2,6-naphthalene dicarboxylic acid amide),poly(metaphenylene-2,6-naphthalene dicarboxylic acid amide),poly(2-chloroparaphenylene terephthalamide), a paraphenyleneterephthalamide/2,6-dichloroparaphenylene terephthalamide copolymer, anda metaphenylene terephthalamide/2,6-dichloroparaphenyleneterephthalamide copolymer. Of the above aramid resins,poly(paraphenylene terephthalamide) is more preferable.

The polyester-based resins are preferably aromatic polyesters such aspolyarylates, and liquid crystal polyesters.

Examples of the rubbers include a styrene-butadiene copolymer and ahydride thereof, a methacrylate ester copolymer, anacrylonitrile-acrylic ester copolymer, a styrene-acrylic estercopolymer, ethylene propylene rubber, and polyvinyl acetate.

Examples of the resins each having a melting point or a glass transitiontemperature of not lower than 180° C. include polyphenylene ether,polysulfone, polyether sulfone, polyphenylene sulfide, polyetherimide,polyamide imide, and polyether amide.

Examples of the water-soluble polymers include polyvinyl alcohol,polyethylene glycol, cellulose ether, sodium alginate, polyacrylic acid,polyacrylamide, and polymethacrylic acid.

Note that the above resins each of which is to be contained in theanother porous layer can be used in one kind or in combination of two ormore kinds.

The other characteristics (e.g., thickness) of the another porous layerare similar to those (of the porous layer) described above, except thatthe porous layer contains the PVDF-based resin.

<Positive Electrode Plate>

The positive electrode plate of the nonaqueous electrolyte secondarybattery in accordance with an embodiment of the present invention is notlimited to any particular positive electrode plate provided that thepositive electrode plate has a capacitance of not less than 1 nF and notmore than 1000 nF per measurement area of 900 mm². For example, as apositive electrode active material layer, a sheet-shaped positiveelectrode plate is used which includes (i) a positive electrode mixcontaining a positive electrode active material, an electricallyconductive agent, and a binding agent and (ii) a positive electrodecurrent collector supporting the positive electrode mix thereon. Notethat the positive electrode plate can be arranged such that the positiveelectrode current collector supports positive electrode mixes onrespective both surfaces of the positive electrode current collector orcan be alternatively arranged such that the positive electrode currentcollector supports the positive electrode mix on one surface of thepositive electrode current collector.

Examples of the positive electrode active material include materialseach capable of being doped with and dedoped of lithium ions. Specificexamples of such a material include a lithium complex oxide containingat least one of transition metals such as V, Mn, Fe, Co, and Ni.

Examples of the electrically conductive agent include carbonaceousmaterials such as natural graphite, artificial graphite, cokes, carbonblack, pyrolytic carbons, carbon fiber, and a fired product of anorganic polymer compound. These electrically conductive agents can beused in one kind or in combination of two or more kinds.

Examples of the binding agent include thermoplastic resins such aspolyvinylidene fluoride, a copolymer of vinylidene fluoride,polytetrafluoroethylene, a tetrafluoroethylene-hexafluoropropylenecopolymer, a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer,an ethylene-tetrafluoroethylene copolymer, a vinylidenefluoride-hexafluoropropylene copolymer, a vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene copolymer, athermoplastic polyimide, polyethylene, and polypropylene; acrylicresins; and styrene-butadiene rubber. The binding agent functions alsoas a thickening agent.

Examples of the positive electrode current collector include electricconductors such as Al, Ni, and stainless steel. Of these electricconductors, Al is more preferable because Al is easily processed into athin film and is inexpensive.

Examples of a method for producing the sheet-shaped positive electrodeplate include a method in which a positive electrode active material, anelectrically conductive agent, and a binding agent are pressure-moldedon a positive electrode current collector; and a method in which (i) apositive electrode active material, an electrically conductive agent,and a binding agent are formed into a paste with use of a suitableorganic solvent, (ii) a positive electrode current collector is coatedwith the paste, and then (iii) the paste is dried and then pressured soas to be firmly fixed to the positive electrode current collector.

<Negative Electrode Plate>

The negative electrode plate of the nonaqueous electrolyte secondarybattery in accordance with an embodiment of the present invention is notlimited to any particular negative electrode plate provided that thenegative electrode plate has a capacitance of not less than 4 nF and notmore than 8500 nF per measurement area of 900 mm². For example, as anegative electrode active material layer, a sheet-shaped negativeelectrode plate is used which includes (i) a negative electrode mixcontaining a negative electrode active material and (ii) a negativeelectrode current collector supporting the negative electrode mixthereon. The sheet-shaped negative electrode plate preferably containsthe above electrically conductive agent and the above binding agent.Note that the negative electrode plate can be arranged such that thenegative electrode current collector supports negative electrode mixeson respective both surfaces of the negative electrode current collectoror can be alternatively arranged such that the negative electrodecurrent collector supports the negative electrode mix on one surface ofthe negative electrode current collector.

Examples of the negative electrode active material include (i) materialseach capable of being doped with and dedoped of lithium ions, (ii)lithium metals, and (iii) lithium alloys. Examples of such a materialinclude a carbonaceous material. Examples of the carbonaceous materialinclude natural graphite, artificial graphite, cokes, carbon black, andpyrolytic carbons. The sheet-shaped negative electrode plate can containan electrically conductive agent and a binding agent which are mentionedas the electrically conductive agent and the binding agent,respectively, each of which can be contained in the positive electrodeactive material layer.

Examples of the negative electrode current collector include electricconductors such as Cu, Ni, and stainless steel. Of these electricconductors, Cu is preferable because Cu is not easily alloyed withlithium particularly in a lithium-ion secondary battery and is easilyprocessed into a thin film.

Examples of a method for producing the sheet-shaped negative electrodeplate include a method in which a negative electrode active material ispressure-molded on a negative electrode current collector; and a methodin which (i) a negative electrode active material is formed into a pastewith use of a suitable organic solvent, (ii) a negative electrodecurrent collector is coated with the paste, and then (iii) the paste isdried and then pressured so as to be firmly fixed to the negativeelectrode current collector. The paste preferably contains theelectrically conductive agent and the binding agent.

<Nonaqueous Electrolyte>

A nonaqueous electrolyte that can be contained in the nonaqueouselectrolyte secondary battery in accordance with an embodiment of thepresent invention is not limited to any particular nonaqueouselectrolyte provided that the nonaqueous electrolyte is a nonaqueouselectrolyte for common use in a nonaqueous electrolyte secondarybattery. The nonaqueous electrolyte can be, for example, a nonaqueouselectrolyte containing an organic solvent and a lithium salt dissolvedin the organic solvent. Examples of the lithium salt include LiClO₄,LiPF₆, LiAsF₆, LiSbF₆, LiBF₄, LiCF₃SO₃, LiN(CF₃SO₂)₂, LiC(CF₃SO₂)₃,Li₂B₁₀Cl₁₀, lower aliphatic carboxylic acid lithium salt, and LiAlCl₄.These lithium salts can be used in one kind or in combination of two ormore kinds.

Examples of the organic solvent which is contained in the nonaqueouselectrolyte include carbonates, ethers, esters, nitriles, amides,carbamates, and sulfur-containing compounds, and fluorine-containingorganic solvents each obtained by introducing a fluorine group into anyof these organic solvents. These organic solvents can be used in onekind or in combination of two or more kinds.

<Method for Producing Nonaqueous Electrolyte Secondary Battery>

The nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention can be produced by, for example, (i)forming a member for a nonaqueous electrolyte secondary battery(hereinafter referred to as a nonaqueous electrolyte secondary batterymember) by providing the positive electrode plate, the porous layer, thenonaqueous electrolyte secondary battery separator, and the negativeelectrode plate in this order, (ii) placing the nonaqueous electrolytesecondary battery member in a container which is to serve as a housingof the nonaqueous electrolyte secondary battery, (iii) filling thecontainer with a nonaqueous electrolyte, and then (iv) hermeticallysealing the container under reduced pressure.

As described earlier, a nonaqueous electrolyte secondary battery inaccordance with an embodiment of the present invention includes: anonaqueous electrolyte secondary battery separator including apolyolefin porous film; a porous layer; a positive electrode plate; anda negative electrode plate. In particular, the nonaqueous electrolytesecondary battery in accordance with an embodiment of the presentinvention satisfies the following requirements (i) through (iv):

-   (i) the requirement that the porous film has a piercing strength of    not less than 26.0 gf/g/m², the piercing strength being measured    with respect to a weight per unit area of the polyolefin porous    film, and the polyolefin porous film has a value in a range of 0.00    to 0.54, the value being represented by the following Expression    (1):

|1−T/M|  (1)

-   (ii) the requirement that a polyvinylidene fluoride-based resin    contained in the porous layer contains an α-form polyvinylidene    fluoride-based resin in an amount of not less than 35.0 mol % with    respect to 100 mol % of a total amount of the α-form polyvinylidene    fluoride-based resin and a β-form polyvinylidene fluoride-based    resin contained in the porous layer;-   (iii) the requirement that the positive electrode plate has a    capacitance of not less than 1 nF and not more than 1000 nF per    measurement area of 900 mm²; and-   (iv) the requirement that the negative electrode plate has a    capacitance of not less than 4 nF and not more than 8500 nF per    measurement area of 900 mm².

According to the nonaqueous electrolyte secondary battery in accordancewith an embodiment of the present invention which nonaqueous electrolytesecondary battery satisfies the requirement (i), it is possible to notonly sufficiently prevent a short circuit between the positive electrodeand the negative electrode but also restrain (a) deformation, caused bycharge and discharge of the nonaqueous electrolyte secondary battery, ofa pore structure inside the nonaqueous electrolyte secondary batteryseparator and (b) deformation, caused by charge and discharge of thenonaqueous electrolyte secondary battery, of a structure of an interfacebetween the nonaqueous electrolyte secondary battery separator or thenonaqueous electrolyte secondary battery laminated separator and theelectrode.

Furthermore, according to the nonaqueous electrolyte secondary batteryin accordance with an embodiment of the present invention whichnonaqueous electrolyte secondary battery satisfies the requirement (ii),the porous layer shows excellent structural stability after thenonaqueous electrolyte secondary battery is charged and discharged undera high rate condition.

Moreover, satisfaction of the requirements (iii) and (iv) allows both apolarization state of the positive electrode active material layer ofthe positive electrode plate and a polarization state of the negativeelectrode active material layer of the negative electrode plate to besuitable. Satisfaction of the requirements (iii) and (iv) also makes itpossible to promote solvation of cations to an electrolyte solvent inthe negative electrode plate and in a place where the negative electrodeplate and the nonaqueous electrolyte secondary battery separator are incontact with each other, and to promote desolvation of the cations fromthe electrolyte solvent in the positive electrode plate and in a placewhere the positive electrode plate and the nonaqueous electrolytesecondary battery separator are in contact with each other. Thisenhances cation permeability.

Thus, the nonaqueous electrolyte secondary battery which satisfies theabove requirements (i) through (iv), (a) makes it possible to restraindeformation, caused by charge and discharge of the nonaqueouselectrolyte secondary battery, of a structure of an interface betweenthe nonaqueous electrolyte secondary battery separator or the nonaqueouselectrolyte secondary battery laminated separator and the electrode, andconsequently to restrain a deterioration in performance of thenonaqueous electrolyte secondary battery. Furthermore, the nonaqueouselectrolyte secondary battery which satisfies the above requirements (i)through (iv) (b) allows the porous layer to show excellent structuralstability after the nonaqueous electrolyte secondary battery is chargedand discharged under a high rate condition, and also (c) allows both thepolarization state of the positive electrode active material layer ofthe positive electrode plate and the polarization state of the negativeelectrode active material layer of the negative electrode plate to besuitable. This allows the nonaqueous electrolyte secondary battery inaccordance with an embodiment of the present invention to have a highercharge capacity during charge at 1 C after high-rate battery discharge(discharge at 20 C).

The present invention is not limited to the embodiments, but can bealtered by a skilled person in the art within the scope of the claims.The present invention also encompasses, in its technical scope, anyembodiment derived by combining technical means disclosed in differingembodiments.

EXAMPLES

The following description will more specifically discuss the presentinvention with reference to Examples and Comparative Examples. Note,however, that the present invention is not limited to the Examples.

[Measurement Method]

Measurements were carried out in Examples and Comparative Examples bythe method below.

(1) Thickness of Active Material Layer (Unit: μm)

A thickness of each a positive electrode active material layer and anegative electrode active material layer was measured with use of ahigh-resolution digital measuring device (VL-50) manufactured byMitutoyo Corporation. Note, however, that the thickness of the positiveelectrode active material layer was calculated by subtracting athickness of aluminum foil, serving as a current collector, from athickness of the positive electrode plate. Note also that the thicknessof the negative electrode active material layer was calculated bysubtracting a thickness of copper foil, serving as a current collector,from a thickness of the negative electrode plate.

(2) Piercing Strength with Respect to Weight Per Unit Area of PorousFilm (Unit: gf/(g/m²))

A porous film material was fixed with a washer of 12 mmφ by use of ahandy-type compression tester (manufactured by KATO TECH CO., LTD.;model number KES-G5). A maximum stress (gf) obtained by piercing theporous film material with a pin at 200 mm/min was defined as a piercingstrength of the porous film material. The pin used had a pin diameter of1 mmφ and a tip radius of 0.5 R.

(3) Scratch Test

A critical load value and a ratio of a critical load distance in atransverse direction to a critical load distance in a machine directionwere measured by a scratch test. Any conditions and the like that aredifferent from the conditions described above and under which to carryout the measurement are similar to those disclosed in JIS R 3255. Themeasurement was carried out with use of a microscratch testing device(manufactured by CSEM Instruments).

-   (3-1) Porous films produced in Examples and Comparative Examples    were each cut into a piece of 20 mm×60 mm. Then, the piece of the    porous film and a glass preparation of 30 mm×70 mm were combined    with use of aqueous glue. Then, the piece and the glass preparation    thus combined were dried at a temperature of 25° C. for one whole    day and night, so that a test sample was produced. Note that the    piece of the porous film and the glass preparation were combined, by    thinly applying, to the piece of the porous film, a small amount of    aqueous glue that would not permeate into the piece of the porous    film, so that no air bubbles were made between the piece of the    porous film and the glass preparation.-   (3-2) The test sample prepared in the step (3-1) was placed on a    microscratch testing device (manufactured by CSEM Instruments).    Then, while a diamond indenter of the testing device was applying a    vertical load of 0.1 N to the test sample, a table of the testing    device was moved by a distance of 10 mm in a transverse direction of    the porous film at a speed of 5 mm/min. During the movement of the    table, a stress (force of friction) that occurred between the    diamond indenter and the test sample was measured.-   (3-3) A line graph was made which shows a relationship between    displacement of the stress measured in the step (3-2) and the    distance of the movement of the table. Then, based on the line    graph, (i) a critical load value in the transverse direction    and (ii) a distance in the transverse direction at which distance a    critical load is reached were calculated.-   (3-4) The direction in which the table is moved was changed to a    machine direction, and the above steps (3-1) through (3-3) were    repeatedly carried out so that (i) a critical load value in the    machine direction and (ii) a distance in the machine direction at    which distance a critical load is reached were calculated.

(4) α Ratio Calculation Method

A laminated body piece having a size of approximately 2 cm×5 cm was cutout from a laminated body produced in each of Examples and ComparativeExamples below. In accordance with the steps (1) through (4) of theabove (Method of calculating respective percentages of α-form PVDF-basedresin and β-form PVDF-based resin each contained in PVDF-based resin), apercentage (α ratio) of an α-form PVDF-based resin contained in aPVDF-based resin was measured.

(5) Measurement of Capacitance of Electrode Plate

A capacitance per measurement area of 900 mm² of each of a positiveelectrode plate and a negative electrode plate, each obtained in each ofExamples and Comparative Examples, was measured with use of an LCR meter(model number: IM3536) manufactured by HIOKI E.E. CORPORATION. Themeasurement was carried out at a frequency of 300 KHz and underconditions set as follows: CV: 0.010 V, SPEED: SLOW2, AVG: 8, CABLE: 1m, OPEN: All, SHORT: All DCBIAS 0.00 V. An absolute value of thecapacitance thus measured was regarded as a capacitance in Examples andComparative Examples.

From an electrode plate which was a measurement target, a single piecewas cut off so that the single piece had (i) a first part which had a 3cm×3 cm square shape and on which an electrode mix was disposed and (ii)a second part which had a 1 cm×1 cm square shape and on which noelectrode mix was disposed. To the second part of the single piece thuscut off from the electrode plate, a lead wire having a length of 6 cmand a width of 0.5 cm was ultrasonically welded so that an electrodeplate whose capacitance was to be measured was obtained (FIG. 1). Analuminum lead wire was used for the positive electrode plate, and anickel lead wire was used for the negative electrode plate.

From a current collector, a single piece was cut off so that the singlepiece had (i) a first part which had a 5 cm×4 cm rectangular shape and(ii) a second part which had a 1 cm×1 cm square shape and to which alead wire was to be welded. To the second part of the single piece thuscut off from the current collector, a lead wire having a length of 6 cmand a width of 0.5 cm was ultrasonically welded so that a probeelectrode (measurement electrode) was obtained (FIG. 2). An aluminumprobe electrode having a thickness of 20 μm was used to measure thecapacitance of the positive electrode plate, and a copper probeelectrode having a thickness of 20 μm was used to measure thecapacitance of the negative electrode plate.

The probe electrode was laid on top of the first part (part having a 3cm×3 cm square shape) of the electrode plate, whose capacitance was tobe measured, so that a laminated body was produced. The laminated bodythus obtained was sandwiched between two sheets of silicon rubber. Aresultant laminated body was further sandwiched between two SUS platesunder a pressure of 0.7 MPa so that a laminated body which was to besubjected to the measurement was obtained. The lead wire of theelectrode plate, whose capacitance was to be measured, and the lead wireof the probe electrode were drawn outside the laminated body which wasto be subjected to the measurement. Each of a voltage terminal and anelectric current terminal of the LCR meter was connected to those leadwires so that the voltage terminal was closer to the electrode platethan the electric current terminal.

(6) Measurement of Porosity of Positive Electrode Active Material Layer

A porosity of a positive electrode active material layer included in apositive electrode plate of Example 1 below was measured by the methodbelow. A porosity of a positive electrode active material layer includedin each of the other positive electrode plates of the other Examplesbelow was also measured by a similar method.

A positive electrode plate in which a positive electrode mix (a mixtureof LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂, an electrically conductive agent, andPVDF (at a weight ratio of 92:5:3)) was disposed on one surface of apositive electrode current collector (aluminum foil) was cut so that apiece having a size of 14.5 cm² (4.5 cm×3 cm+1 cm×1 cm) was obtained. Aresultant cut piece of the positive electrode plate had a mass of 0.215g and a thickness of 58 μm. The positive electrode current collector wascut so that a piece having the same size as the cut piece of thepositive electrode plate was obtained. A resultant cut piece of thepositive electrode current collector had a mass of 0.078 g and athickness of 20 μm.

A density ρ of such a positive electrode active material layer wascalculated as (0.215−0.078)/{(58−20)/10000×14.5}=2.5 g/cm³.

Each of materials contained in the positive electrode mix had a truedensity as below. Specifically, the LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂ had atrue density of 4.68 g/cm³, the electrically conductive agent had a truedensity of 1.8 g/cm³, and the PVDF had a true density of 1.8 g/cm³.

The positive electrode active material layer had a porosity ε of 40%,which was calculated, from the above values, based on the followingexpression:

ε=[1−{2.5×(92/100)/4.68+2.5×(5/100)/1.8+2.5×(3/100)/1.8}]×100=40%

(7) Measurement of Porosity of Negative Electrode Active Material Layer

A porosity of a negative electrode active material layer included in anegative electrode plate of Example 1 below was measured by the methodbelow. A porosity of a negative electrode active material layer includedin each of the other negative electrode plates of the other Examplesbelow was also measured by a similar method.

A negative electrode plate in which a negative electrode mix (a mixtureof graphite, a styrene-1,3-butadiene copolymer, and sodium carboxymethylcellulose (at a weight ratio of 98:1:1)) was disposed on one surface ofa negative electrode current collector (copper foil) was cut so that apiece having a size of 18.5 cm² (5 cm×3.5 cm+1 cm×1 cm) was obtained. Aresultant cut piece of the negative electrode plate had a mass of 0.266g and a thickness of 48 μm. The negative electrode current collector wascut so that a piece having the same size as the cut piece of thenegative electrode plate was obtained. A resultant cut piece of thenegative electrode current collector had a mass of 0.162 g and athickness of 10 μm.

A density ρ of such a negative electrode active material layer wascalculated as (0.266−0.162)/{(48−10) /10000×18.5}=1.49 g/cm³.

Each of materials contained in the negative electrode mix had a truedensity as below. Specifically, the graphite had a true density of 2.2g/cm³, the styrene-1,3-butadiene copolymer had a true density of 1g/cm³, and the sodium carboxymethyl cellulose had a true density of 1.6g/cm³.

The negative electrode active material layer had a porosity ε of 31%,which was calculated, from the above values, based on the followingexpression:

ε=[1−{1.49×(98/100)/2.2+1.49×(1/100)/1+1.49'(1/100)/1.6}]×100=31%

(8) Battery Characteristic of Nonaqueous Electrolyte Secondary Battery

A charge capacity characteristic obtained after a nonaqueous electrolytesecondary battery (having a design capacity of 20.5 mAh) produced ineach of Examples and Comparative Examples had been subjected high-ratedischarge was measured by a method including the following steps (A) and(B).

(A) Initial Charge and Discharge Test

A new nonaqueous electrolyte secondary battery which had been producedin each of Examples and Comparative Examples and which had not beensubjected to any charge and discharge cycle was subjected to 4 initialcharge and discharge cycles at 25° C. Each of the 4 initial charge anddischarge cycles was carried out under the condition that a voltageranged from 2.7 V to 4.1 V, CC-CV charge was carried out at a chargecurrent value of 0.2 C (terminal current condition: 0.02 C), and CCdischarge was carried out at a discharge current value of 0.2 C. Notethat 1 C is defined as a value of an electric current at which a ratedcapacity based on a discharge capacity at 1 hour rate is discharged in 1hour. Same applies to the following description. Note that the CC-CVcharge is a charging method in which a battery is charged at a setconstant electric current, and after a given voltage is reached, thegiven voltage is maintained while the electric current is reduced. Notealso that the CC discharge is a discharging method in which a battery isdischarged at a set constant electric current until a given voltage isreached. Same applies to the following description.

(B) Charge Capacity Characteristic After High-Rate Discharge (mAh)

The nonaqueous electrolyte secondary battery which had been subjected tothe initial charge and discharge was subjected to charge and dischargecycles under the condition that (i) a temperature was set at 55° C.,(ii) CC-CV charge was carried out at a rate of 1 C (final rate: 0.02 C),and (iii) CC discharge was carried out such that a rate was set to 0.2 Cfirst and then the rate was changed to 1 C, 5 C, and 10 C in this orderevery three charge and discharge cycles. The charge and discharge cycleswere carried out at a voltage in a range of 2.7 V to 4.2 V.

A charge capacity obtained during charge at 1 C in the third one of thethree charge and discharge cycles in which discharge at 20 C was carriedout was regarded as a charge capacity after high-rate discharge (mAh),which is shown in Table 1.

Example 1

[Production of Nonaqueous Electrolyte Secondary Battery LaminatedSeparator]

Ultra-high molecular weight polyethylene powder (GUR4032, manufacturedby Ticona Corporation) having a weight-average molecular weight of4,970,000 and polyethylene wax (FNP-0115, manufactured by Nippon SeiroCo., Ltd.) having a weight-average molecular weight of 1,000 were mixedso as to prepare a mixture containing the ultra-high molecular weightpolyethylene powder in an amount of 70% by weight and the polyethylenewax in an amount of 30% by weight. Assuming that the ultra-highmolecular weight polyethylene powder and the polyethylene wax of themixture had 100 parts by weight in total, to the 100 parts by weight ofthe mixture, 0.4 parts by weight of an antioxidant (Irg1010,manufactured by Ciba Specialty Chemicals Inc.), 0.1 parts by weight ofan antioxidant (P168, manufactured by Ciba Specialty Chemicals Inc.),and 1.3 parts by weight of sodium stearate were added, and then calciumcarbonate (manufactured by Maruo Calcium Co., Ltd.) having an averageparticle diameter of 0.1 μm was further added so as to account for 36%by volume of a total volume of a resultant mixture. Then, the resultantmixture was mixed as it was, that is, in a form of powder, in a Henschelmixer, so that a mixture 1 was obtained.

Then, the mixture 1 was melted and kneaded in a twin screw kneadingextruder, so that a polyolefin resin composition 1 was obtained. Then,the polyolefin resin composition 1 was rolled with use of a roller at acircumferential velocity of 3.0 m/min, so that a rolled sheet wasproduced. Subsequently, the rolled sheet 1 was immersed in an aqueoushydrochloric acid solution (containing 4 mol/L of hydrochloric acid and0.5% by weight of a nonionic surface active agent) so that the calciumcarbonate was removed from the rolled sheet 1. Then, the rolled sheetwas stretched at 105° C. at a stretch ratio of 6.2 times (ratio ofstretch temperature to stretch ratio=16.9). Furthermore, the rolledsheet was subjected to heat fixation at 120° C., so that a porous film 1was obtained. The porous film 1 obtained had a weight per unit area of6.9 g/m².

An N-methyl-2-pyrrolidone (hereinafter also referred to as “NMP”)solution (manufactured by Kureha Corporation; product name: “L #9305”,weight-average molecular weight: 1,000,000) containing a PVDF-basedresin (polyvinylidene fluoride-hexafluoropropylene copolymer) wasprepared as a coating solution 1. The coating solution 1 was applied tothe porous film 1 by a doctor blade method so that the PVDF-based resincontained in the coating solution 1 which had been applied to the porousfilm 1 weighed 6.0 g per square meter of the porous film 1.

A resultant coated product was immersed in 2-propanol while a coatedfilm thereof was wet with a solvent contained in the coating solution,and then was left to stand still at −10° C. for 5 minutes, so that alaminated porous film 1 was obtained. The laminated porous film 1obtained was further immersed in other 2-propanol while being wet withthe above immersion solvent, and then was left to stand still at 25° C.for 5 minutes, so that a laminated porous film 1 a was obtained. Thelaminated porous film 1 a obtained was dried at 30° C. for 5 minutes, sothat a laminated separator 1 was produced. Table 1 shows results ofevaluation of the laminated separator 1 produced.

[Production of Nonaqueous Electrolyte Secondary Battery]

(Production of Positive Electrode Plate)

A positive electrode plate was used which had been produced by applyinga mixture of LiNi_(0.5)Mn_(0.3)Co_(0.2)O₂, an electrically conductiveagent, and PVDF (at a weight ratio of 92:5:3) to aluminum foil. Thealuminum foil of the positive electrode plate was cut off so that a cutpiece of the aluminum foil had (i) a first part, on which a positiveelectrode active material layer was disposed, had a size of 45 mm×30 mmand (ii) a second part, on which no positive electrode active materiallayer was disposed, had a width of 13 mm and remained so as to surroundthe first part. A positive electrode plate 1 was thus obtained. Thepositive electrode active material layer had a thickness of 38 μm and adensity of 2.50 g/cm³.

(Production of Negative Electrode Plate)

A negative electrode plate was used which had been produced by applyinga mixture of graphite, a styrene-1,3-butadiene copolymer, and sodiumcarboxymethylcellulose (at a weight ratio of 98:1:1) to copper foil.

The copper foil of the negative electrode plate was cut off so that acut piece of the copper foil had (i) a first part, on which a negativeelectrode active material layer was disposed, had a size of 50 mm×35 mmand (ii) a second part, on which no negative electrode active materiallayer was disposed, had a width of 13 mm and remained so as to surroundthe first part. A negative electrode plate 1 was thus obtained. Thenegative electrode active material layer had a thickness of 38 μm and adensity of 1.49 g/cm³.

(Assembly of Nonaqueous Electrolyte Secondary Battery)

The positive electrode plate 1, the negative electrode plate 1, and thelaminated separator 1 were used to produce a nonaqueous electrolytesecondary battery by the following method.

(i) The positive electrode plate 1, (ii) the laminated separator 1including a porous layer provided so as to face the positive electrodeplate 1, and (iii) the negative electrode plate 1 were disposed on topof each other (provided) in this order in a laminate pouch, so that anonaqueous electrolyte secondary battery member 1 was obtained. In thiscase, the positive electrode plate 1 and the negative electrode plate 1were provided so that a whole of a main surface of the positiveelectrode active material layer of the positive electrode plate 1 wasincluded in a range of a main surface (overlapped the main surface) ofthe negative electrode active material layer of the negative electrodeplate 1.

Subsequently, the nonaqueous electrolyte secondary battery member 1 wasput into a bag which had been made, in advance, of a laminate of analuminum layer and a heat seal layer. Further, 0.25 mL of nonaqueouselectrolyte was put into the bag. The above nonaqueous electrolyte wasprepared by dissolving LiPF₆ in a mixed solvent of ethylene carbonate,ethyl methyl carbonate, and diethyl carbonate at a ratio (volume ratio)of 3:5:2 so that the LiPF₆ was contained at 1 mol/L. Then, the bag washeat-sealed while the pressure inside the bag was reduced, so that anonaqueous electrolyte secondary battery 1 was produced.

Thereafter, a charge capacity after high-rate discharge of thenonaqueous electrolyte secondary battery 1 was measured by the methoddescribed in the above (8) Battery characteristic of nonaqueouselectrolyte secondary battery. Table 1 shows results of the measurement.

Example 2

[Production of Nonaqueous Electrolyte Secondary Battery LaminatedSeparator]

Example 2 prepared a polyolefin resin composition 2 as in the case ofExample 1 except that Example 2 (i) used (a) ultra-high molecular weightpolyethylene powder (GUR4032, manufactured by Ticona Corporation),having a weight-average molecular weight of 4,970,000, in an amount of72% by weight and (b) polyethylene wax (FNP-0115, manufactured by NipponSeiro Co., Ltd.), having a weight-average molecular weight of 1000, inan amount of 29% by weight and (ii) used calcium carbonate (manufacturedby Maruo Calcium Co., Ltd.), having an average pore size of 0.1 μm, sothat the calcium carbonate accounted for 37% by amount of a total volumeof a resultant mixture.

Subsequently, the polyolefin resin composition 2 was rolled with use ofa roller at a circumferential velocity of 4.0 m/min, so that a rolledsheet 2 was produced. Then, Example 2 obtained a porous film 2 as in thecase of Example 1 by removing the calcium carbonate from the rolledsheet 2, stretching the rolled sheet 2, and subjecting the rolled sheet2 to heat fixation, except that Example 2 set the stretch temperature at100° C., set the stretch ratio at 7.0 times (ratio of stretchtemperature to stretch ratio=16.9), and carried out heat fixation at123° C. The porous film 2 obtained had a weight per unit area of 5.4g/m².

Example 2 applied the coating solution 1 to the porous film 2 as in thecase of Example 1. A resultant coated product was immersed in 2-propanolwhile a coated film thereof was wet with a solvent contained in thecoating solution, and then was left to stand still at −5° C. for 5minutes, so that a laminated porous film 2 was obtained. The laminatedporous film 2 obtained was further immersed in other 2-propanol whilebeing wet with the above immersion solvent, and then was left to standstill at 25° C. for 5 minutes, so that a laminated porous film 2 a wasobtained. The laminated porous film 2 a obtained was dried at 30° C. for5 minutes, so that a laminated separator 2 was produced. Table 1 showsresults of evaluation of the laminated separator 2 produced.

[Production of Nonaqueous Electrolyte Secondary Battery]

Example 2 produced a nonaqueous electrolyte secondary battery as in thecase of Example 1 except that Example 2 used the laminated separator 2instead of the laminated separator 1. The nonaqueous electrolytesecondary battery thus produced was designated as a nonaqueouselectrolyte secondary battery 2.

Thereafter, a charge capacity after high-rate discharge of thenonaqueous electrolyte secondary battery 2 was measured by the methoddescribed in the above (8) Battery characteristic of nonaqueouselectrolyte secondary battery. Table 1 shows results of the measurement.

Example 3

(Production of Positive Electrode Plate)

A surface of a positive electrode plate, identical to the positiveelectrode plate 1, which surface was located on the positive electrodeactive material layer side was rubbed 3 times with use of an abrasivecloth sheet (model number: TYPE AA GRIT No. 100) manufactured byNagatsuka Abrasive Mfg. A positive electrode plate 2 was thus obtained.The positive electrode active material layer of the positive electrodeplate 2 had a thickness of 38 μm and a porosity of 40%.

[Production of Nonaqueous Electrolyte Secondary Battery]

Example 3 used the negative electrode plate 1 as a negative electrodeplate. Example 3 produced a nonaqueous electrolyte secondary battery asin the case of Example 1 except that Example 3 used the laminatedseparator 2 instead of the laminated separator 1 and used the positiveelectrode plate 2 as a positive electrode plate. The nonaqueouselectrolyte secondary battery thus produced was designated as anonaqueous electrolyte secondary battery 3.

Thereafter, a charge capacity after high-rate discharge of thenonaqueous electrolyte secondary battery 3 was measured by the methoddescribed in the above (8) Battery characteristic of nonaqueouselectrolyte secondary battery. Table 1 shows results of the measurement.

Example 4

(Production of Positive Electrode Plate)

A surface of a positive electrode plate, identical to the positiveelectrode plate 1, which surface was located on the positive electrodeactive material layer side was rubbed 5 times with use of an abrasivecloth sheet (model number: TYPE AA GRIT No. 100) manufactured byNagatsuka Abrasive Mfg. A positive electrode plate 3 was thus obtained.The positive electrode active material layer of the positive electrodeplate 3 had a thickness of 38 μm and a porosity of 40%.

[Production of Nonaqueous Electrolyte Secondary Battery]

Example 4 used the negative electrode plate 1 as a negative electrodeplate. Example 4 produced a nonaqueous electrolyte secondary battery asin the case of Example 1 except that Example 4 used the laminatedseparator 2 instead of the laminated separator 1 and used the positiveelectrode plate 3 as a positive electrode plate. The nonaqueouselectrolyte secondary battery thus produced was designated as anonaqueous electrolyte secondary battery 4.

Thereafter, a charge capacity after high-rate discharge of thenonaqueous electrolyte secondary battery 4 was measured by the methoddescribed in the above (8) Battery characteristic of nonaqueouselectrolyte secondary battery. Table 1 shows results of the measurement.

Example 5

(Production of Negative Electrode Plate)

A surface of a negative electrode plate, identical to the negativeelectrode plate 1, which surface was located on the negative electrodeactive material layer side was rubbed 3 times with use of an abrasivecloth sheet (model number: TYPE AA GRIT No. 100) manufactured byNagatsuka Abrasive Mfg. A negative electrode plate 2 was thus obtained.The negative electrode active material layer of the negative electrodeplate 2 had a thickness of 38 μm and a porosity of 31%.

[Production of Nonaqueous Electrolyte Secondary Battery]

The positive electrode plate 1 was used as a positive electrode plate.Example 5 produced a nonaqueous electrolyte secondary battery as in thecase of Example 1 except that Example 5 used the laminated separator 2instead of the laminated separator 1 and used the negative electrodeplate 2 as a negative electrode plate. The nonaqueous electrolytesecondary battery thus produced was designated as a nonaqueouselectrolyte secondary battery 5.

Thereafter, a charge capacity after high-rate discharge of thenonaqueous electrolyte secondary battery 5 was measured by the methoddescribed in the above (8) Battery characteristic of nonaqueouselectrolyte secondary battery. Table 1 shows results of the measurement.

Example 6

(Production of Negative Electrode Plate)

A surface of a negative electrode plate, identical to the negativeelectrode plate 1, which surface was located on the negative electrodeactive material layer side was rubbed 7 times with use of an abrasivecloth sheet (model number: TYPE AA GRIT No. 100) manufactured byNagatsuka Abrasive Mfg. A negative electrode plate 3 was thus obtained.The negative electrode active material layer of the negative electrodeplate 3 had a thickness of 38 μm and a porosity of 31%.

[Production of Nonaqueous Electrolyte Secondary Battery]

The positive electrode plate 1 was used as a positive electrode plate.Example 6 produced a nonaqueous electrolyte secondary battery as in thecase of Example 1 except that Example 6 used the laminated separator 2instead of the laminated separator 1 and used the negative electrodeplate 3 as a negative electrode plate. The nonaqueous electrolytesecondary battery thus produced was designated as a nonaqueouselectrolyte secondary battery 6.

Thereafter, a charge capacity after high-rate discharge of thenonaqueous electrolyte secondary battery 6 was measured by the methoddescribed in the above (8) Battery characteristic of nonaqueouselectrolyte secondary battery. Table 1 shows results of the measurement.

Example 7

[Production of Porous Layer and Production of Laminated Separator]

A PVDF-based resin (manufactured by Arkema Inc.; product name “Kynar(Registered Trademark) LBG”, having a weight-average molecular weight of590,000) was dissolved, by being stirred at 65° C. over 30 minutes, inN-methyl-2-pyrrolidone so that a solid content in a resultant solutionwas 10% by mass. The resultant solution was used as a binder solution.As a filler, alumina fine particles (manufactured by Sumitomo ChemicalCo., Ltd.; product name “AKP3000”, containing 5 ppm of silicon) wereused. The alumina fine particles, the binder solution, and a solvent(N-methyl-2-pyrrolidone) were mixed together at the following ratio.That is, the alumina fine particles, the binder solution, and thesolvent were mixed together so that (i) a resultant mixed solutioncontained 10 parts by weight of the PVDF-based resin with respect to 90parts by weight of the alumina fine particles and (ii) a concentrationof a solid content (alumina fine particles+PVDF-based resin) in themixed solution was 10% by weight. A dispersion solution (coatingsolution 2) was thus obtained.

The coating solution was applied to the porous film 2, which had beenproduced in Example 2, by a doctor blade method so that the PVDF-basedresin contained in the coating solution 2 which had been applied to theporous film 2 weighed 6.0 g per square meter of the porous film 2. Alaminated porous film 3 was thus produced. The laminated porous film 3was dried at 65° C. for 5 minutes, so that a laminated separator 3 wasproduced. The laminated porous film was dried by hot air blownperpendicularly to a base material at an air velocity of 0.5 m/s. Table1 shows results of evaluation of the laminated separator 3 produced.

[Production of Nonaqueous Electrolyte Secondary Battery]

Example 7 produced a nonaqueous electrolyte secondary battery as in thecase of Example 1 except that Example 7 used the laminated separator 3instead of the laminated separator 1. The nonaqueous electrolytesecondary battery thus produced was designated as a nonaqueouselectrolyte secondary battery 7.

Thereafter, a charge capacity after high-rate discharge of thenonaqueous electrolyte secondary battery 7 was measured by the methoddescribed in the above (8) Battery characteristic of nonaqueouselectrolyte secondary battery. Table 1 shows results of the measurement.

Comparative Example 1

(Production of Positive Electrode Plate)

A surface of a positive electrode plate, identical to the positiveelectrode plate 1, which surface was located on the positive electrodeactive material layer side was rubbed 10 times with use of an abrasivecloth sheet (model number: TYPE AA GRIT No. 100) manufactured byNagatsuka Abrasive Mfg. A positive electrode plate 4 was thus obtained.The positive electrode active material layer of the positive electrodeplate 4 had a thickness of 38 μm and a porosity of 40%.

[Production of Nonaqueous Electrolyte Secondary Battery]

Comparative Example 1 used the negative electrode plate 1 as a negativeelectrode plate. Comparative Example 1 produced a nonaqueous electrolytesecondary battery as in the case of Example 1 except that ComparativeExample 1 used the laminated separator 2 as a nonaqueous electrolytesecondary battery separator and used the positive electrode plate 4 as apositive electrode plate. The nonaqueous electrolyte secondary batterythus produced was designated as a nonaqueous electrolyte secondarybattery 8.

Thereafter, a charge capacity after high-rate discharge of thenonaqueous electrolyte secondary battery 8 was measured by the methoddescribed in the above (8) Battery characteristic of nonaqueouselectrolyte secondary battery. Table 1 shows results of the measurement.

Comparative Example 2

(Production of Negative Electrode Plate)

A surface of a negative electrode plate, identical to the negativeelectrode plate 1, which surface was located on the negative electrodeactive material layer side was rubbed 10 times with use of an abrasivecloth sheet (model number: TYPE AA GRIT No. 100) manufactured byNagatsuka Abrasive Mfg. A negative electrode plate 4 was thus obtained.The negative electrode active material layer of the negative electrodeplate 4 had a thickness of 38 μm and a porosity of 31%.

[Production of Nonaqueous Electrolyte Secondary Battery]

The positive electrode plate 1 was used as a positive electrode plate.Comparative Example 2 produced a nonaqueous electrolyte secondarybattery as in the case of Example 1 except that Comparative Example 2used the laminated separator 2 as a laminated separator and used thenegative electrode plate 4 as a negative electrode plate. The nonaqueouselectrolyte secondary battery thus produced was designated as anonaqueous electrolyte secondary battery 9.

Thereafter, a charge capacity after high-rate discharge of thenonaqueous electrolyte secondary battery 9 was measured by the methoddescribed in the above (8) Battery characteristic of nonaqueouselectrolyte secondary battery. Table 1 shows results of the measurement.

Comparative Example 3

[Production of Nonaqueous Electrolyte Secondary Battery Separator]

A coated product obtained as in the case of the obtainment of the coatedproduct in Example 2 was immersed in 2-propanol while a coated filmthereof was wet with a solvent contained in a coating solution, and thenwas left to stand still at −78° C. for 5 minutes, so that a laminatedporous film 4 was obtained. The laminated porous film 4 obtained wasfurther immersed in other 2-propanol while being wet with the aboveimmersion solvent, and then was left to stand still at 25° C. for 5minutes, so that a laminated porous film 4 a was obtained. The laminatedporous film 4 a obtained was dried at 30° C. for 5 minutes, so that alaminated separator 4 was produced. Table 1 shows results of evaluationof the laminated separator 4 produced.

[Production of Nonaqueous Electrolyte Secondary Battery]

Comparative Example 3 produced a nonaqueous electrolyte secondarybattery as in the case of Example 1 except that Comparative Example 3used the laminated separator 4 as a nonaqueous electrolyte secondarybattery separator. The nonaqueous electrolyte secondary battery thusproduced was designated as a nonaqueous electrolyte secondary battery10.

Thereafter, a charge capacity after high-rate discharge of thenonaqueous electrolyte secondary battery 10 was measured by the methoddescribed in the above (8) Battery characteristic of nonaqueouselectrolyte secondary battery. Table 1 shows results of the measurement.

TABLE 1 Battery Laminated separator Electrode Charging Positive Negativecharacteristic Porous film Porous electrode electrode Charge capacityPiercing layer plate plate after high- strength PVDFα ratio CapacitanceCapacitance rate discharge (gf/(g/m²)) |1 − TD/MD| (mol %) (nF) (nF)(mAh) Ex. 1 52.5 0.42 35.3 2.1 4.7 12.4 Ex. 2 64.1 0.37 44.4 2.1 4.711.0 Ex. 3 64.1 0.37 44.4 60 4.7 10.7 Ex. 4 64.1 0.37 44.4 935 4.7 11.8Ex. 5 64.1 0.37 44.4 2.1 274 11.3 Ex. 6 64.1 0.37 44.4 2.1 7400 10.9 Ex.7 64.1 0.37 64.3 2.1 4.7 11.3 Comp. Ex. 1 64.1 0.37 44.4 4090 4.7 6.4Comp. Ex. 2 64.1 0.37 44.4 2.1 9050 4.9 Comp. Ex. 3 64.1 0.37 34.6 2.14.7 5.2 *“Ex.” stands for “Example”, and “Comp. Ex.” stands for“Comparative Example.”

Table 1 shows that, as compared with the nonaqueous electrolytesecondary battery produced in each of Comparative Examples 1 to 3, thenonaqueous electrolyte secondary battery produced in each of Examples 1to 7 had a more excellent charge capacity characteristic after high-ratedischarge.

This reveals that a nonaqueous electrolyte secondary battery whichsatisfies the four requirements described earlier in <Method forproducing nonaqueous electrolyte secondary battery> can have an enhancedcharge capacity characteristic after high-rate discharge.

INDUSTRIAL APPLICABILITY

A nonaqueous electrolyte secondary battery in accordance with anembodiment of the present invention is excellent in charge capacitycharacteristic after high-rate discharge. Thus, the nonaqueouselectrolyte secondary battery in accordance with an embodiment of thepresent invention can be suitably used as a battery for, for example,any of a personal computer, a mobile telephone, a portable informationterminal, and a vehicle.

REFERENCE SIGNS LIST

1 Diamond indenter

2 Substrate

3 Porous film

1. A nonaqueous electrolyte secondary battery comprising: a nonaqueouselectrolyte secondary battery separator including a polyolefin porousfilm; a porous layer containing a polyvinylidene fluoride-based resin; apositive electrode plate having a capacitance of not less than 1 nF andnot more than 1000 nF per measurement area of 900 mm²; and a negativeelectrode plate having a capacitance of not less than 4 nF and not morethan 8500 nF per measurement area of 900 mm², the polyolefin porous filmhaving a piercing strength of not less than 26.0 gf/g/m², the piercingstrength being measured with respect to a weight per unit area of thepolyolefin porous film, the polyolefin porous film having a value in arange of 0.00 to 0.54, the value being represented by the followingExpression (1):|1−T/M|  (1) where T represents a distance at which a critical load isreached in a scratch test in which the polyolefin porous film is movedin a transverse direction under a constant load of 0.1 N, and Mrepresents a distance at which a critical load is reached in a scratchtest in which the polyolefin porous film is moved in a machine directionunder a constant load of 0.1 N, the porous layer being provided betweenthe nonaqueous electrolyte secondary battery separator and at least oneof the positive electrode plate and the negative electrode plate, andthe polyvinylidene fluoride-based resin containing an α-formpolyvinylidene fluoride-based resin in an amount of not less than 35.0mol % with respect to 100 mol % of a total amount of the α-formpolyvinylidene fluoride-based resin and a β-form polyvinylidenefluoride-based resin contained in the polyvinylidene fluoride-basedresin, where the amount of the α-form polyvinylidene fluoride-basedresin contained is calculated from waveform separation of (α/2) observedat around −78 ppm in a ¹⁹F-NMR spectrum obtained from the porous layer,and waveform separation of {(α/2)+P} observed at around −95 ppm in the¹⁹F-MMR spectrum obtained from the porous layer.
 2. The nonaqueouselectrolyte secondary battery as set forth in claim 1, wherein thepositive electrode plate contains a transition metal oxide.
 3. Thenonaqueous electrolyte secondary battery as set forth in claim 1,wherein the negative electrode plate contains graphite.
 4. Thenonaqueous electrolyte secondary battery as set forth in claim 1,further comprising: another porous layer which is provided between (i)the nonaqueous electrolyte secondary battery separator and (ii) at leastone of the positive electrode plate and the negative electrode plate. 5.The nonaqueous electrolyte secondary battery as set forth in claim 4,wherein the another porous layer contains at least one resin selectedfrom the group consisting of a polyolefin, a (meth)acrylate-based resin,a fluorine-containing resin (excluding a polyvinylidene fluoride-basedresin), a polyamide-based resin, a polyester-based resin, and awater-soluble polymer.
 6. The nonaqueous electrolyte secondary batteryas set forth in claim 5, wherein the polyamide-based resin is an aramidresin.